Na(+)-K(+)-ATPase alpha 1- and beta 1-subunit degradation: evidence for multiple subunit specific rates

1993 ◽  
Vol 264 (3) ◽  
pp. C583-C590 ◽  
Author(s):  
L. Lescale-Matys ◽  
D. S. Putnam ◽  
A. A. McDonough
Keyword(s):  
Beta Subunit ◽  
Beta Subunits ◽  
Plasma Membrane Protein ◽  
Significant Component ◽  
Chase Period ◽  
Pig Kidney ◽  
Alpha Subunits ◽  
Degradation Rates ◽  
Synthesis And Degradation ◽  
Rate Of Accumulation

Na(+)-K(+)-ATPase is a heterodimeric plasma membrane protein consisting of an alpha-catalytic and a beta-glycoprotein subunit. Because these two subunits are derived from two separate genes, they may not be synthesized with stoichiometric equivalence. The aim of this study was to estimate relative rates of synthesis and degradation of nascent and mature Na(+)-K(+)-ATPase alpha- and beta-subunits to determine whether either of the nascent subunits accumulates in excess and, if so, the fate of the excess subunits. We studied a pig kidney cell line (LLC-PK1/Cl4) that expresses only alpha 1- and beta 1-subunits. Relative synthesis and degradation rates of nascent subunits were first estimated by pulsing cells for 10 min with [35S]methionine followed by chase periods of up to 120 min and by immunoprecipitation. We found that directly after labeling, beta-subunits were present in threefold excess over alpha-subunits and that nearly 50% of this beta-subunit pool was degraded by 60 min. Nascent alpha-subunits were not degraded during the chase period. In a second strategy to examine relative rates of nascent alpha- vs. beta-subunit accumulation, cells were pulsed for 5-60 min and immunoprecipitated directly (without chase). The rate of accumulation of labeled alpha was greater than that of beta between 5 and 60 min, consistent with the results of the pulse-chase strategy, demonstrating a significant component of degradation of beta during this period. Despite the very different degradation rates of newly synthesized alpha- vs. beta-subunits, the degradation rates of alpha- and beta-subunits beyond 4 h after synthesis were indistinguishable (t0.5 = 10-12 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of subunit assembly of the Na-K-ATPase

1994 ◽  
Vol 266 (3) ◽  
pp. C579-C589 ◽  
Author(s):  
D. M. Fambrough ◽  
M. V. Lemas ◽  
M. Hamrick ◽  
M. Emerick ◽  
K. J. Renaud ◽  
...  
Keyword(s):  
Mammalian Cells ◽  
Sodium Pump ◽  
Beta Subunit ◽  
Alpha Subunit ◽  
Beta Subunits ◽  
Limited Region ◽  
Subunit Assembly ◽  
Alpha Subunits ◽  
Subunit Isoforms ◽  
P Type

The Na-K-ATPase, or sodium pump, is comprised of two subunits, alpha and beta. Each subunit spans the lipid bilayer of the cell membrane. This review summarizes our efforts to determine how the two subunits interact to form the functional ion transporter. Our major approach has been to observe the potential for subunit assembly when one or both subunits are truncated or present as chimeras that retain only a limited region of the Na-K-ATPase. DNAs encoding these altered subunit forms of the avian Na-K-ATPase are expressed in mammalian cells. Monoclonal antibodies specific for the avian beta-subunit are then used to purify newly synthesized avian beta-subunits, and the presence of accompanying alpha-subunits indicates that subunit assembly has occurred. The ectodomain of the beta-subunit (approximately residues 62-304) is sufficient for assembly with the alpha-subunit, and a COOH-terminal truncation of the beta-subunit that lacks aminoacyl residues beyond 162 will assemble inefficiently. A maximum of 26 aminoacyl residues of the alpha-subunit are necessary for robust assembly with the beta-subunit, when this sequence replaces the COOH-terminal half of the loop between membrane spans 7 and 8 in the SERCA1 Ca-ATPase. This region of the Ca-ATPase faces the lumen of the endoplasmic reticulum. These findings encourage study of other related questions, including whether there is preferential assembly of certain subunit isoforms and how various P-type ATPases are targeted to their appropriate subcellular compartments.

scholarly journals Functional reconstitution of the human interleukin-3 receptor

Blood ◽  
1992 ◽  
Vol 80 (1) ◽  
pp. 84-90 ◽  
Author(s):  
T Kitamura ◽  
A Miyajima
Keyword(s):  
Ligand Binding ◽  
Growth Response ◽  
Human Factor ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Stable Transfectants ◽  
Interleukin 3 ◽  
Gm Csf ◽  
High Affinity ◽  
Alpha Subunits

The high-affinity receptors for human interleukin-3 (IL-3), GM-CSF, and IL-5 are composed of alpha and beta subunits. The alpha subunits are primary ligand binding proteins specific for each ligand, whereas the three human receptors share a common beta subunit (beta c). In contrast to humans mice have two closely related genes, AIC2A and AIC2B, which are homologous to human beta c. The AIC2A gene encodes a low-affinity murine IL-3 binding protein, and the AIC2B protein is the beta subunit shared between murine GM-CSF receptors (mGMR) and IL-5 receptors (mIL- 5R). To examine the function of these receptor components, we established various stable transfectants of murine IL-2-dependent CTLL- 2 cells. CTLL-2 transfectants expressing both the alpha and beta subunits of the human IL-3 receptor (hIL-3R) proliferated in response to physiologic concentrations of hIL-3. Coexpression of hIL-3R alpha with AIC2B but not with AIC2A in CTLL-2 cells conferred a growth response to hIL-3. Although CTLL-2 transfectants expressing hIL-3R alpha alone did not proliferate in the presence of hIL-3, hIL-3- responsive sublines were repeatedly isolated. These sublines expressed endogenous AIC2B but not AIC2A. These results indicate that human beta c is essential for hIL-3 signaling and that AIC2B is a murine equivalent of human beta c. We also showed that hIL-3 and hGM-CSF induced tyrosine phosphorylation of several proteins in CTLL transfectants, similar to those observed in human factor-dependent TF-1 cells stimulated with hIL-3 and hGM-CSF.

scholarly journals Functional reconstitution of the human interleukin-3 receptor

Blood ◽  
1992 ◽  
Vol 80 (1) ◽  
pp. 84-90 ◽  
Author(s):  
T Kitamura ◽  
A Miyajima
Keyword(s):  
Ligand Binding ◽  
Growth Response ◽  
Human Factor ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Stable Transfectants ◽  
Interleukin 3 ◽  
Gm Csf ◽  
High Affinity ◽  
Alpha Subunits

Abstract The high-affinity receptors for human interleukin-3 (IL-3), GM-CSF, and IL-5 are composed of alpha and beta subunits. The alpha subunits are primary ligand binding proteins specific for each ligand, whereas the three human receptors share a common beta subunit (beta c). In contrast to humans mice have two closely related genes, AIC2A and AIC2B, which are homologous to human beta c. The AIC2A gene encodes a low-affinity murine IL-3 binding protein, and the AIC2B protein is the beta subunit shared between murine GM-CSF receptors (mGMR) and IL-5 receptors (mIL- 5R). To examine the function of these receptor components, we established various stable transfectants of murine IL-2-dependent CTLL- 2 cells. CTLL-2 transfectants expressing both the alpha and beta subunits of the human IL-3 receptor (hIL-3R) proliferated in response to physiologic concentrations of hIL-3. Coexpression of hIL-3R alpha with AIC2B but not with AIC2A in CTLL-2 cells conferred a growth response to hIL-3. Although CTLL-2 transfectants expressing hIL-3R alpha alone did not proliferate in the presence of hIL-3, hIL-3- responsive sublines were repeatedly isolated. These sublines expressed endogenous AIC2B but not AIC2A. These results indicate that human beta c is essential for hIL-3 signaling and that AIC2B is a murine equivalent of human beta c. We also showed that hIL-3 and hGM-CSF induced tyrosine phosphorylation of several proteins in CTLL transfectants, similar to those observed in human factor-dependent TF-1 cells stimulated with hIL-3 and hGM-CSF.

scholarly journals Insulin and insulin-like-growth-factor-I (IGF-I) receptors in Xenopus laevis oocytes. Comparison with insulin receptors from liver and muscle

1991 ◽  
Vol 273 (3) ◽  
pp. 673-678 ◽  
Author(s):  
P Hainaut ◽  
A Kowalski ◽  
S Giorgetti ◽  
V Baron ◽  
E Van Obberghen
Keyword(s):  
Growth Factor ◽  
Xenopus Laevis ◽  
Insulin Receptors ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Xenopus Laevis Oocytes ◽  
Insulin Like Growth Factor ◽  
Factor I ◽  
Alpha Subunits ◽  
Igf I

Insulin and insulin-like-growth-factor-I (IGF-I) receptors were partially purified from full-grown (stages V-VI) Xenopus laevis oocytes by affinity chromatography on wheat-germ agglutinin-agarose. Competitive-binding assays revealed high-affinity binding sites for both insulin and IGF-I (Kd = 2.5 x 10(-10) M and 8 x 10(-10) M respectively). However, IGF-I receptors were about 15 times more abundant than insulin receptors (22.5 x 10(11) versus 1.5 x 10(11)/mg of protein). Moreover, comparison of intact and collagenase-treated oocytes showed that most of the insulin receptors were in the oocyte envelopes, whereas IGF-I receptors were essentially at the oocyte surface. Oocyte receptors were composed of alpha-subunits of approximately 130 kDa and a doublet of beta-subunits of 95 and 105 kDa, which both had ligand-induced phosphorylation patterns compatible with IGF-I receptor beta-subunits. Accordingly, the receptor tyrosine kinase was stimulated at low IGF-I concentrations [half-maximally effective concentration (EC50) approximately 0.5-1 nM], and at higher insulin concentrations (EC50 approximately 20-50 nM). Partially purified glycoproteins from Xenopus liver and muscle contained mainly receptors of the insulin-receptor type, with alpha-subunits of 140 kDa in liver and 125 kDa in muscle, and doublets of beta-subunits of 92-98 kDa in liver and 85-94 kDa in muscle. Immunoprecipitation of receptors from oocytes, liver and muscle by receptor-specific anti-peptide antibodies suggested that the beta-subunit heterogeneity resulted from the existence of two distinct IGF-I receptors in oocytes and of two distinct insulin receptors in both liver and muscle. In the different tissues, the two receptor subtypes differed at least by their beta-subunit C-terminal region.

scholarly journals A human leukocyte differentiation antigen family with distinct alpha-subunits and a common beta-subunit: the lymphocyte function-associated antigen (LFA-1), the C3bi complement receptor (OKM1/Mac-1), and the p150,95 molecule.

1983 ◽  
Vol 158 (6) ◽  
pp. 1785-1803 ◽  
Author(s):  
F Sanchez-Madrid ◽  
J A Nagy ◽  
E Robbins ◽  
P Simon ◽  
T A Springer
Keyword(s):  
Human Leukocyte ◽  
Beta Subunit ◽  
Alpha Subunit ◽  
Beta Subunits ◽  
Complement Receptor ◽  
Lymphocyte Function ◽  
Subunit Dissociation ◽  
Molecular Weights ◽  
Alpha Subunits ◽  
Sds Page

The human lymphocyte function-associated antigen-1 (LFA-1), the complement receptor-associated OKM1 molecule, and a previously undescribed molecule termed p150,95, have been found to be structurally and antigenically related. Each antigen contains an alpha- and beta-subunit noncovalently associated in an alpha 1 beta 1-structure as shown by cross-linking experiments. LFA-1, OKM1, and p150,95 alpha-subunit designations and their molecular weights are alpha L = 177,000 Mr, alpha M = 165,000 Mr, and alpha X = 150,000 Mr, respectively. The beta-subunits are all = 95,000 Mr. Some MAb precipitated only LFA-1, others only OKM1, and another precipitates all three antigens. The specificity of these MAb for particular subunits was examined after subunit dissociation by high pH. MAb specific for LFA-1 or OKM1 bind to the alpha L- or alpha M-subunits, respectively, while the cross-reactive MAb binds to the beta-subunits. Coprecipitation experiments with intact alpha 1 beta 1-complexes showed anti-alpha and anti-beta MAb can precipitate the same molecules. In two-dimensional (2D) isoelectric focusing-SDS-PAGE, the alpha subunits of the three antigens are distinct, while the beta-subunits are identical. Biosynthesis experiments showed alpha L, alpha M, and alpha X are synthesized from distinct precursors, as is beta. The three antigens differ in expression on lymphocytes, granulocytes, and monocytes. During maturation of the monoblast-like U937 line, alpha M and alpha X are upregulated and alpha L is downregulated. Some MAb to the alpha subunit of OKM1 inhibited the complement receptor type three. LFA-1, OKM1, and p150,95 constitute a novel family of functionally important human leukocyte antigens that share a common beta-subunit.

scholarly journals Protein kinase activity of the insulin receptor

1986 ◽  
Vol 235 (1) ◽  
pp. 1-11 ◽  
Author(s):  
S Gammeltoft ◽  
E Van Obberghen
Keyword(s):  
Insulin Receptor ◽  
Molecular Mechanisms ◽  
Kinase Activity ◽  
Beta Subunit ◽  
Alpha Subunit ◽  
Beta Subunits ◽  
Protein Kinase Activity ◽  
Intact Cells ◽  
Receptor Complexes ◽  
Alpha Subunits

The insulin receptor is an integral membrane glycoprotein (Mr approximately 300,000) composed of two alpha-subunits (Mr approximately 130,000) and two beta-subunits (Mr approximately 95,000) linked by disulphide bonds. This oligomeric structure divides the receptor into two functional domains such that alpha-subunits bind insulin and beta-subunits possess tyrosine kinase activity. The amino acid sequence deduced from cDNA of the single polypeptide chain precursor of human placental insulin receptor revealed that alpha- and beta-subunits consist of 735 and 620 residues, respectively. The alpha-subunit is hydrophilic, disulphide-bonded, glycosylated and probably extracellular. The beta-subunit consists of a short extracellular region which links the alpha-subunit through disulphide bridges, a hydrophobic transmembrane region and a longer cytoplasmic region which is structurally homologous with other tyrosine kinases like the src oncogene product and EGF receptor kinases. The cellular function of insulin receptors is dual: transmembrane signalling and endocytosis of hormone. The binding of insulin to its receptor on the cell membrane induces transfer of signal from extracellular to cytoplasmic receptor domains leading to activation of cell metabolism and growth. In addition, hormone-receptor complexes are internalized leading to intracellular proteolysis of insulin, whereas receptors are recycled to the membrane. These phenomena are kinetically well-characterized, but their molecular mechanisms remain obscure. Insulin receptor in different tissues and animal species are homologous in their structure and function, but show also significant differences regarding size of alpha-subunits, binding kinetics, insulin specificity and receptor-mediated degradation. We suggest that this heterogeneity of receptors may be linked to the diversity in insulin effects on metabolism and growth in various cell types. The purified insulin receptor phosphorylates its own beta-subunit and exogenous protein and peptide substrates on tyrosine residues, a reaction which is insulin-sensitive, Mn2+-dependent and specific for ATP. Tyrosine phosphorylation of the beta-subunit activates receptor kinase activity, and dephosphorylation with alkaline phosphatase deactivates the kinase. In intact cells or impure receptor preparations, a serine kinase is also activated by insulin. The cellular role of two kinase activities associated with the insulin receptor is not known, but we propose that the tyrosine- and serine-specific kinases mediate insulin actions on metabolism and growth either through dual-signalling or sequential pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Characterization of beta pat-3 heterodimers, a family of essential integrin receptors in C. elegans.

1995 ◽  
Vol 129 (4) ◽  
pp. 1127-1141 ◽  
Author(s):  
S N Gettner ◽  
C Kenyon ◽  
L F Reichardt
Keyword(s):  
Caenorhabditis Elegans ◽  
Cytoplasmic Domain ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Integrin Receptors ◽  
Diverse Range ◽  
C Elegans ◽  
Alpha Subunits ◽  
Sds Page

Members of the integrin family of cell surface receptors have been shown to mediate a diverse range of cellular functions that require cell-cell or cell-extracellular matrix interactions. We have initiated the characterization of integrin receptors from the nematode Caenorhabditis elegans, an organism in which genetics can be used to study integrin function with single cell resolution. Here we report the cloning of an integrin beta subunit from C. elegans which is shown to rescue the embryonic lethal mutation pat-3(rh54) and is thus named beta pat-3. Analysis of the deduced amino acid sequence revealed that beta pat-3 is more similar to Drosophila integrin beta PS and to vertebrate integrin beta 1 than to other integrin beta subunits. Regions of highest homology are in the RGD-binding region and in the cytoplasmic domain. In addition, the 56 cysteines present in the majority of integrin beta subunits are conserved. A major transcript of approximately 3 kilo-base pairs was detected by RNA blot analysis. Immunoblot analysis using a polyclonal antiserum against the cytoplasmic domain showed that beta pat-3 migrates in SDS-PAGE with apparent M(r) of 109 k and 120 k under nonreducing and reducing conditions, respectively. At least nine protein bands with relative molecular weights in the range observed for known integrin alpha subunits coprecipitate with beta pat-3, and at least three of these bands migrate in SDS-PAGE with increased mobility when reduced. This behavior has been observed for a majority of integrin alpha subunits. Immunoprecipitations of beta pat-3 from developmentally staged populations of C. elegans showed that the expression of several of these bands changes during development. The monoclonal antibody MH25, which has been postulated to recognize the transmembrane component of the muscle dense body structure a (Francis, G. R., and R. H. Waterston. 1985. Muscle organization in Caenorhabditis elegans: localization of proteins implicated in thin filament attachment and I-band organization. J. Cell Biol. 101:1532-1549), was shown to recognize beta pat-3. Finally, immunocytochemical analysis revealed that beta pat-3 is expressed in the embryo and in many cell types postembryonically, including muscle, somatic gonad, and coelomocytes, suggesting multiple roles for integrin heterodimers containing this beta subunit in the developing animal.

scholarly journals Inherited deficiency of the Mac-1, LFA-1, p150,95 glycoprotein family and its molecular basis.

1984 ◽  
Vol 160 (6) ◽  
pp. 1901-1918 ◽  
Author(s):  
T A Springer ◽  
W S Thompson ◽  
L J Miller ◽  
F C Schmalstieg ◽  
D C Anderson
Keyword(s):  
Monoclonal Antibodies ◽  
Cell Surface ◽  
Molecular Basis ◽  
Autosomal Recessive Inheritance ◽  
Surface Expression ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Normal Surface ◽  
Alpha Subunits ◽  
Deceased Patient

Leukocyte surface glycoproteins that share a common beta subunit have been found to be congenitally deficient in three unrelated patients with recurring bacterial infection. The glycoproteins, Mac-1, LFA-1, and p150,95, have the subunit compositions alpha M beta, alpha L beta, and alpha X beta, respectively. Using subunit-specific monoclonal antibodies, both the alpha M and beta subunits of Mac-1, the alpha L and beta subunits of LFA-1, and at the least the beta subunit of p150,95, were found to be deficient at the cell surface by the techniques of immunofluorescence flow cytometry, radioimmunoassay, and immunoprecipitation. A latent pool of Mac-1 that can be expressed on granulocyte surfaces in response to secretory stimuli, such as f-Met-Leu-Phe, was also lacking in patients. Deficiency was found on all leukocytes tested, including granulocytes, monocytes, and T and B lymphocytes. Quantitation by immunofluorescence cytometry of subunits on granulocytes from parents of these patients and of a fourth deceased patient showed approximately half-normal surface expression, and, together with data on other siblings and a family with an affected father and children, demonstrate autosomal recessive inheritance. Deficiency appears to be quantitative rather than qualitative, with two patients expressing approximately 0.5% and one patient approximately 5% of normal amounts. The latter patient had alpha beta complexes on the cell surface detectable by immunoprecipitation. Biosynthesis experiments showed the presence of normal amounts of alpha'L intracellular precursor in lymphoid lines of all three patients. Together with surface deficiency of three molecules that share a common beta subunit but have differing alpha subunits, this suggests the primary deficiency is of the beta subunit. The lack of maturation of alpha'L to alpha L and the deficiency of the alpha subunits at the cell surface and in latent pools suggests that association with the beta subunit is required for alpha subunit processing and transport to the cell surface or to latent pools. The molecular basis of this disease is discussed in light of adhesion-related functional abnormalities in patients' leukocytes and the blockade of similar functions in healthy cells by monoclonal antibodies.

scholarly journals Insulin stimulates proteolysis of the α-subunit, but not the β-subunit, of its receptor at the cell surface in rat liver

1989 ◽  
Vol 261 (2) ◽  
pp. 333-340 ◽  
Author(s):  
K E Lipson ◽  
A A Kolhatkar ◽  
D B Donner
Keyword(s):  
Cell Surface ◽  
Rat Liver ◽  
Plasma Membranes ◽  
Polyacrylamide Gel Electrophoresis ◽  
Beta Subunit ◽  
Beta Subunits ◽  
Α Subunit ◽  
Receptor Complexes ◽  
Alpha Subunits ◽  
Alpha Beta

Insulin receptors in rat liver plasma membranes contain two alpha- and two beta-subunits held together by interchain disulphide bonds ([alpha beta]2 receptors). Affinity-labelled receptors were digested with chymotrypsin or elastase and then exposed to dithiothreitol before solubilization from membranes and SDS/polyacrylamide-gel electrophoresis. This resulted in partial reduction and isolation of Mr-225,000 alpha beta, Mr-200,000 alpha 1 beta, Mr-165,000 alpha beta 1 and Mr-145,000 alpha 1 beta 1 receptor halves containing intact (alpha, beta) or degraded (alpha 1, beta 1) subunits. The ability to identify half-receptor complexes containing intact or degraded subunits made it possible to assay each subunit simultaneously for insulin-induced proteolysis in isolated plasma membranes or during perfusion of rat liver in situ with insulin. In liver membranes, insulin binding increased the fraction of receptors containing degraded alpha-subunits to about one-third of the total population during 2 h of incubation at 23 degrees C. beta-Subunit proteolysis increased only minimally during this time. Plasma membranes isolated from livers perfused with insulin at 37 degrees C contained degraded alpha-subunits but only intact beta-subunits, showing that insulin induced cell-surface proteolysis of the binding, but not the kinase, domain of its receptor. Since previous observations [Lipson, Kolhatkar & Donner (1988) J. Biol. Chem 263, 10495-10501] have shown that receptors containing degraded alpha-subunits are internalized but do not recycle, it is possible that cell-surface degradation may play a role in the regulation of insulin-receptor number in hepatic tissue. Proteolysis of the beta-subunit is not a likely mechanism by which receptor-kinase activity may be attenuated under physiological conditions.

scholarly journals Mapping of antigenic and functional epitopes on the alpha- and beta-subunits of two related mouse glycoproteins involved in cell interactions, LFA-1 and Mac-1.

1983 ◽  
Vol 158 (2) ◽  
pp. 586-602 ◽  
Author(s):  
F Sanchez-Madrid ◽  
P Simon ◽  
S Thompson ◽  
T A Springer
Keyword(s):  
Monoclonal Antibodies ◽  
Structural Features ◽  
Cell Interactions ◽  
Beta Subunit ◽  
Alpha Subunit ◽  
Beta Subunits ◽  
Complement Receptor ◽  
Receptor Function ◽  
Alpha Subunits ◽  
Sds Page

Mouse Mac-1, a complement receptor-associated surface structure on macrophages, and LFA-1, a function-associated structure on lymphocytes, comprise a novel family of leukocyte differentiation antigens participating in adhesive cell interactions. Mac-1 and LFA-1 contain alpha-subunits of 170,000 and 180,000 Mr, respectively, and beta-subunits of 95,000 Mr noncovalently associated in alpha 1 beta 1 complexes. The structural relation between the alpha- and between the beta-subunits, and the location of functionally important sites on the molecules, have been probed with antibodies. Both non-cross-reactive and cross-reactive monoclonal antibodies (MAb) and antisera prepared to the purified molecules or the LFA-1 alpha-subunits were used. Reactivity with individual subunits was studied by immunoprecipitation after dissociation induced by high pH treatment, or by immunoblotting after SDS-PAGE. Cross-reactive epitopes on Mac-1 and LFA-1 were found to be present on the beta-subunits, which were immunologically identical. Non-cross-reactive epitopes that are distinctive for Mac-1 or LFA-1 were localized to the alpha-subunits. MAb to LFA-1 alpha-subunit epitopes inhibited CTL-mediated killing. Two MAb to Mac-1 alpha-subunit epitopes but not a third MAb to a spatially distinct alpha-epitope inhibited complement receptor function. Neither function was inhibited by a MAb binding to a common beta-subunit epitope. Therefore, sites of Mac-1 and LFA-1 involved in their respective adhesion-related functions, as well as distinctive structural features, have been localized to the alpha-subunits.

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