scholarly journals Interchangeable alpha chain cytoplasmic domains play a positive role in control of cell adhesion mediated by VLA-4, a beta 1 integrin.

1993 ◽  
Vol 178 (2) ◽  
pp. 649-660 ◽  
Author(s):  
P D Kassner ◽  
M E Hemler
Keyword(s):  
Cell Adhesion ◽  
Cell Lines ◽  
Deletion Mutant ◽  
K562 Cells ◽  
Cytoplasmic Domain ◽  
Wild Type ◽  
Cytoplasmic Domains ◽  
Positive Role ◽  
Alpha Chain ◽  
Poor Adhesion

Integrins can exist in a range of functional states, depending on the cell type and its state of activation. Although the mechanism that controls activity is unknown, it has been suggested that for some integrins, alpha chain cytoplasmic domains may exert either a negative effect or no effect on adhesion function. To address this issue for VLA-4 (an alpha 4 beta 1 heterodimer), we constructed an alpha 4 cytoplasmic deletion mutant and chimeric alpha chains composed of the extracellular domains of alpha 4 and the cytoplasmic domains of alpha 2, alpha 4, or alpha 5. Upon stable transfection of wild-type alpha 4, VLA-4 heterodimer was obtained that mediated (a) poor adhesion to CS1 peptide, fibronectin, or vascular cell adhesion molecule 1 (VCAM-1) (in K562 cells); (b) poor adhesion to CS1 peptide but moderate adhesion to VCAM-1 (in MIP101 cells); and (c) moderate adhesion to both CS1 peptide and VCAM-1 (in PMWK cells). Chimeric alpha 4 constructs and wild-type alpha 4 yielded similar results in these cell lines. In contrast, truncation of the alpha 4 cytoplasmic domain (after the conserved GFFKR motif) caused an almost complete loss of adhesive activity in all three cell lines. Thus, several interchangeable alpha chain cytoplasmic domains play a fundamentally positive role in determining the state of constitutive activity for VLA-4. The alpha chain cytoplasmic domain is also required for agonist-stimulated adhesion, since phorbol ester stimulated the cell adhesion mediated by wild-type and chimeric alpha chains, but not by the cytoplasmic deletion mutant. The inactivity of both wild-type VLA-4 (in K562 cells), and truncated VLA-4 (in all three cell lines) was overcome by the addition of a stimulatory anti-beta 1 monoclonal antibody. Thus, the alpha cytoplasmic domain-dependent cellular mechanism controlling both constitutive and agonist-stimulated VLA-4 activity could be bypassed by external manipulation of the integrin.

scholarly journals A critical cytoplasmic domain of the interleukin-5 (IL-5) receptor alpha chain and its function in IL-5-mediated growth signal transduction.

1994 ◽  
Vol 14 (11) ◽  
pp. 7404-7413 ◽  
Author(s):  
S Takaki ◽  
H Kanazawa ◽  
M Shiiba ◽  
K Takatsu
Keyword(s):  
Signal Transduction ◽  
Protein Tyrosine Kinase ◽  
Cytoplasmic Domain ◽  
Beta Chain ◽  
Cytoplasmic Domains ◽  
Interleukin 5 ◽  
Alpha Chain ◽  
Gm Csf ◽  
Response Expression ◽  
And Function

Interleukin-5 (IL-5) regulates the production and function of B cells, eosinophils, and basophils. The IL-5 receptor (IL-5R) consists of two distinct membrane proteins, alpha and beta. The alpha chain (IL-5R alpha) is specific to IL-5. The beta chain is the common beta chain (beta c) of receptors for IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF). The cytoplasmic domains of both alpha and beta chains are essential for signal transduction. In this study, we generated cDNAs of IL-5R alpha having various mutations in their cytoplasmic domains and examined the function of these mutants by expressing them in IL-3-dependent FDC-P1 cells. The membrane-proximal proline-rich sequence of the cytoplasmic domain of IL-5R alpha, which is conserved among the alpha chains of IL-5R, IL-3R, and GM-CSF receptor (GM-CSFR), was found to be essential for the IL-5-induced proliferative response, expression of nuclear proto-oncogenes such as c-jun, c-fos, and c-myc, and tyrosine phosphorylation of cellular proteins including JAK2 protein-tyrosine kinase. In addition, analysis using chimeric receptors which consist of the extracellular domain of IL-5R alpha and the cytoplasmic domain of beta c suggested that dimerization of the cytoplasmic domain of beta c may be an important step in activating the IL-5R complex and transducing intracellular growth signals.

scholarly journals Integrin cytoplasmic domains mediate inside-out signal transduction

1994 ◽  
Vol 124 (6) ◽  
pp. 1047-1059 ◽  
Author(s):  
TE O'Toole ◽  
Y Katagiri ◽  
RJ Faull ◽  
K Peter ◽  
R Tamura ◽  
...  
Keyword(s):  
K562 Cells ◽  
Cytoplasmic Domain ◽  
Alpha Subunit ◽  
Affinity Binding ◽  
Cell Type ◽  
Cytoplasmic Domains ◽  
High Affinity Binding ◽  
High Affinity ◽  
Energy Dependent ◽  
Inside Out

We analyzed the binding of fibronectin to integrin alpha 5 beta 1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd approximately 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of alpha IIb beta 3 joined to the cytoplasmic domains of alpha 5 beta 1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of alpha 5 beta 1 conferred an energy-dependent high affinity state on alpha IIb beta 3 in CHO but not K562 cells. Three additional alpha cytoplasmic domains (alpha 2, alpha 6A, alpha 6B) conferred PAC1 binding in CHO cells, while three others (alpha M, alpha L, alpha v) did not. In the high affinity alpha chimeras, cotransfection with a truncated (beta 3 delta 724) or mutated (beta 3(S752-->P)) beta 3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved alpha subunit GFFKR motif, resulted in high affinity binding of ligands to alpha IIb beta 3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the beta subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the alpha subunit cytoplasmic domain maintains the default low affinity state.

scholarly journals Modulation of β1A Integrin Functions by Tyrosine Residues in the β1 Cytoplasmic Domain

1998 ◽  
Vol 141 (2) ◽  
pp. 527-538 ◽  
Author(s):  
Takao Sakai ◽  
Qinghong Zhang ◽  
Reinhard Fässler ◽  
Deane F. Mosher
Keyword(s):  
Stem Cells ◽  
Cell Adhesion ◽  
Point Mutations ◽  
Cytoplasmic Domain ◽  
Wild Type ◽  
Directed Migration ◽  
Tyrosine Residues ◽  
Impaired Ability ◽  
Focal Contacts ◽  
Activating Mutation

β1A integrin subunits with point mutations of the cytoplasmic domain were expressed in fibroblasts derived from β1-null stem cells. β1A in which one or both of the tyrosines of the two NPXY motifs (Y783, Y795) were changed to phenylalanines formed active α5β1 and α6β1 integrins that mediated cell adhesion and supported assembly of fibronectin. Mutation of the proline in either motif (P781, P793) to an alanine or of a threonine in the inter-motif sequence (T788) to a proline resulted in poorly expressed, inactive β1A. Y783,795F cells developed numerous fine focal contacts and exhibited motility on a surface. When compared with cells expressing wild-type β1A or β1A with the D759A activating mutation of a conserved membrane–proximal aspartate, Y783,795F cells had impaired ability to transverse filters in chemotaxis assays. Analysis of cells expressing β1A with single Tyr to Phe substitutions indicated that both Y783 and Y795 are important for directed migration. Actin-containing microfilaments of Y783,795F cells were shorter and more peripheral than microfilaments of cells expressing wild-type β1A. These results indicate that change of the phenol side chains in the NPXY motifs to phenyl groups (which cannot be phosphorylated) has major effects on the organization of focal contacts and cytoskeleton and on directed cell motility.

Essential Role for Activated Leukocyte Cell Adhesion Molecule Gene Silencing by GATA-1 in Megakaryocytic Progenitor Cell Biology.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1194-1194
Author(s):  
Fang Tan ◽  
Robert Thomas ◽  
Flaubert Mbeunkui ◽  
Solomon F. Ofori-Acquah
Keyword(s):  
Cell Adhesion ◽  
Cell Lines ◽  
Adhesion Molecule ◽  
Cell Biology ◽  
Progenitor Cell ◽  
Cell Adhesion Molecule ◽  
K562 Cells ◽  
Jurkat Cells ◽  
Erythroid Progenitor ◽  
Erythroid Progenitor Cell

Abstract Regulation of hematopoietic progenitor cell lineage-commitment, proliferation and differentiation by cell-cell adhesion mechanisms is poorly understood. Activated leukocyte cell adhesion molecule (ALCAM) is a member of the immunoglobulin super family. It is expressed by human hematopoietic stem cells, bone marrow stromal cells, endothelial cells and osteoblasts. Monoclonal anti-ALCAM antibodies inhibit myeloid but not erythroid colony formation, which suggest a lineage-specific role for ALCAM in hematopoiesis. To explore this hypothesis, ALCAM mRNA and protein expression was quantified in human hematopoietic cell lines of myeloid, lymphoid, erythroid, and megakaryocytic lineages by real-time quantitative PCR and western blot analyses. No ALCAM transcripts were detected in K562 and MEG-01 cells, the level of ALCAM mRNA was 2-fold more abundant in HL-60 and THP-1 cells than in U937 and Jurkat cells. This expression pattern was confirmed at the protein level as none of the megakaryocyte-erythroid progenitor cell lines (K562, MEG-01 and HEL) expressed ALCAM. On the contrary, ALCAM was abundantly expressed in THP-1 and HL-60 cells and moderately in U937 and Jurkat cells. GATA-1 was abundantly expressed in megakaryocyte-erythroid progenitor cell lines but not in any of the myeloid cell lines. Thus, there is an inverse relationship between expression of ALCAM and GATA-1 in hematopoietic cells. To test the hypothesis that GATA-1 is involved in silencing ALCAM gene expression, multiple ALCAM-promoter luciferase constructs were studied. A negative regulatory region was identified in the ALCAM promoter containing an inverted GATA-1 cis element at −850 upstream of the translational start site. GATA-1 occupied this canonical element in vivo as determined by chromatin immunoprecipitation experiments. A two-base pair mutation of the −850 GATA-1 cis element increased ALCAM promoter activity 3-fold in K562 and MEG-01 cells, providing direct evidence of GATA-1’s negative regulatory role in ALCAM promoter activity. To test the hypothesis that ALCAM silencing is essential for megakaryocyte-erythroid progenitor cell biology, stable lines of K562 cells were established forcibly expressing ALCAM-GFP or a control GFP. Live cell imaging demonstrated recruitment of ALCAM to sites of cell-cell adhesion in ALCAM-GFP-K562 cells, whereas GFP remained distributed in the cell cytosol in control cells. ALCAM-GFP-K562 cells formed markedly more clusters consisting of significantly more cells than control GFP-K562 cells. Finally, the number of ALCAM-GFP-K562 cells at log-phase growth was significantly higher than GFP-K562 cells over the same time period. Our findings demonstrate for the first time lineage-specific silencing of the cell adhesion molecule ALCAM in megakaryocyte-erythroid progenitor cells, mediated at least in part by GATA-1. That ectopic expression of ALCAM increased proliferation of K562 cells suggests that GATA-1-mediated silencing of ALCAM is essential in slowing down expansion of megakaryocyte-erythroid progenitor cells. Indeed, preliminary studies show an excessive number of erythroid and megakaryocytic cells in the adult spleen of ALCAM-null mice. This model is being used in ongoing studies to confirm our findings in vivo.

Point mutation of C-terminal region of CD20 molecule predicts rituximab-induced complement-dependent cytotoxicity and clinical response to rituximab in non-Hodgkin’s lymphoma

2006 ◽  
Vol 24 (18_suppl) ◽  
pp. 7563-7563 ◽  
Author(s):  
Y. Terui ◽  
Y. Mishima ◽  
Y. Mishima ◽  
M. Yokoyama ◽  
K. Hatake ◽  
...  
Keyword(s):  
Flow Cytometry ◽  
Laser Scanning ◽  
Transmembrane Domain ◽  
Point Mutations ◽  
K562 Cells ◽  
Cytoplasmic Domain ◽  
Laser Scanning Confocal Microscopy ◽  
Cytoplasmic Domains ◽  
Isolated Cells ◽  
Cd20 Expression

7563 Background: Although rituximab is commonly used as induction and maintenance therapy for malignant lymphoma, some patients become refractory to treatment and the mechanism of resistance is unclear. The aim of this study was to investigate the relationship between CD20 mutations and rituximab resistance. Methods: To investigate whether CD20 mutations affect response to rituximab, fresh CD19+ lymphoma cells were isolated from the lymph nodes, or bone marrow of 68 patients with NHL. The cells were subsequently sorted by flow cytometry. RNA was prepared from the isolated cells and RT-PCR was performed. The PCR products were sequenced, subcloned into an expression vector pTARGET, transfected into K562 cells. CD20 expression was examined by flow cytometry and laser scanning confocal microscopy. Results: In all 68 patients, overall response rate (CR+CRu+PR) to rituximab was 91.2% (62/68), but t four cases became PD after PR. DNA sequence analysis revealed that point mutations were mostly observed in three CD20 domains - extracellular/cytoplasmic domains, the third transmembrane domain and the C-terminal cytoplasmic domain. Two cases had point mutations in extracellular/cytoplasmic domains, one patient had point mutations in the transmembrane domain, four cases showed point mutations in the C-terminal cytoplasmic domain and six cases had non-specific CD20 mutations, which did not affect CD20 expression. 56 patients showed no mutations of CD20 gene. CD20 expression was very weak in patients with point mutations in the C-terminal cytoplasmic domain, whereas expression was increased in patients with point mutations in the transmembrane domain. Conclusions: Point mutations in CD20 may cause rituximab resistance and identification of CD20 mutations upon diagnosis may help to predict a patient’s response to rituximab. No significant financial relationships to disclose.

scholarly journals Integrin alpha 2 cytoplasmic domain deletion effects: loss of adhesive activity parallels ligand-independent recruitment into focal adhesions.

1994 ◽  
Vol 5 (9) ◽  
pp. 977-988 ◽  
Author(s):  
S Kawaguchi ◽  
J M Bergelson ◽  
R W Finberg ◽  
M E Hemler
Keyword(s):  
Amino Acids ◽  
Cell Adhesion ◽  
Focal Adhesion ◽  
Cho Cells ◽  
Chinese Hamster ◽  
Focal Adhesions ◽  
Inverse Correlation ◽  
Cytoplasmic Domain ◽  
Negative Regulator ◽  
Wild Type

Chinese hamster ovary (CHO) cells transfected with the integrin alpha 2 subunit formed a stable VLA-2 heterodimer that mediated cell adhesion to collagen. Within CHO cells spread on collagen, but not fibronectin, wild-type alpha 2 subunit localized into focal adhesion complexes (FACs). In contrast, alpha 2 with a deleted cytoplasmic domain was recruited into FACs whether CHO cells were spread on collagen or fibronectin. Thus, as previously seen for other integrins, the alpha 2 cytoplasmic domain acts as a negative regulator, preventing indiscriminate integrin recruitment into FACs. Notably, ligand-independent localization of the VLA-2 alpha 2 subunit into FACs was partially prevented if only one or two amino acids were present in the alpha 2 cytoplasmic domain (beyond the conserved GFFKR motif) and was completely prevented by four to seven amino acids. The addition of two alanine residues (added to GFFKR) also partially prevented ligand-independent localization. In a striking inverse correlation, the same mutants showing increased ligand-independent recruitment into FACs exhibited diminished alpha 2-dependent adhesion to collagen. Thus, control of VLA-2 localization may be closely related to the suppression of cell adhesion to collagen. In contrast to FAC localization and collagen adhesion results, VLA-2-dependent binding and infection by echovirus were unaffected by either alpha 2 cytoplasmic domain deletion or exchange with other cytoplasmic domains.

scholarly journals PPM1F controls integrin activity via a conserved phospho-switch

2020 ◽  
Vol 219 (12) ◽  
Author(s):  
Tanja M. Grimm ◽  
Nina I. Dierdorf ◽  
Karin Betz ◽  
Christoph Paone ◽  
Christof R. Hauck
Keyword(s):  
Cell Adhesion ◽  
Cell Lines ◽  
Cytoplasmic Tail ◽  
Cytoplasmic Domain ◽  
Genetic Screen ◽  
Tissue Homeostasis ◽  
Integrin Β1 ◽  
Gain Of Function ◽  
Novel Target ◽  
Embryonic Death

Control of integrin activity is vital during development and tissue homeostasis, while derailment of integrin function contributes to pathophysiological processes. Phosphorylation of a conserved threonine motif (T788/T789) in the integrin β cytoplasmic domain increases integrin activity. Here, we report that T788/T789 functions as a phospho-switch, which determines the association with either talin and kindlin-2, the major integrin activators, or filaminA, an integrin activity suppressor. A genetic screen identifies the phosphatase PPM1F as the critical enzyme, which selectively and directly dephosphorylates the T788/T789 motif. PPM1F-deficient cell lines show constitutive integrin phosphorylation, exaggerated talin binding, increased integrin activity, and enhanced cell adhesion. These gain-of-function phenotypes are reverted by reexpression of active PPM1F, but not a phosphatase-dead mutant. Disruption of the ppm1f gene in mice results in early embryonic death at day E10.5. Together, PPM1F controls the T788/T789 phospho-switch in the integrin β1 cytoplasmic tail and constitutes a novel target to modulate integrin activity.

The Novel Sulfonamidebenzamide ML291 Activates Apoptotic UPR Signaling in Pediatric Leukemia

Blood ◽  
2016 ◽  
Vol 128 (22) ◽  
pp. 3523-3523
Author(s):  
Danielle Garshott ◽  
Nicole Melong ◽  
Tania T. Sarker ◽  
Yue Xi ◽  
Amy Brownell ◽  
...  
Keyword(s):  
Cell Lines ◽  
Leukemia Cell ◽  
Leukemic Cell ◽  
K562 Cells ◽  
Leukemia Cells ◽  
Wild Type ◽  
Leukemia Cell Lines ◽  
Proliferation Assays

Abstract Background: Acute leukemias are the most common cancers in childhood. Despite multi-agent chemotherapy protocols and the introduction of novel molecularly targeted therapies which have resulted in improved survival over the last few decades, relapsed acute lymphoblastic leukemia remains the second most common pediatric cancer diagnosis. In addition, morbidities from current chemotherapy regimens are unacceptably high. Abundant evidence point to a major role for mediators of the unfolded protein response (UPR) in normal and leukemic white blood cell biology. We have demonstrated that activation of the UPR is a productive approach to inhibit the proliferation of solid tumor cell lines in vitro and to reducing xenograft burden in vivo. The UPR consists of genetically distinct mechanisms that serve to clear misfolded proteins from the endoplasmic reticulum (ER) and enhance protein folding, or induce apoptosis if the initiating stress is prolonged or robust. ML291 is a novel UPR-inducing sulfonamidebenzamide, identified through cell-based high throughput screening and iterative SAR-guided chemical synthesis, that overwhelms the adaptive capacity of the UPR and induces apoptosis in a variety of solid cancer models. Objective: To determine the ability of ML291 to activate the UPR and induce apoptosis in a panel of leukemia cell lines, and to use CHOP-null K562 cells to elucidate the relative contribution of the UPR. We hypothesized that ML291 might activate the PERK/eIF2a/CHOP (apoptotic) arm of the UPR and reduce leukemic cell burden in vitro and in vivo. Methods: MTT and luciferase-based proliferation assays, flow cytometry and RT-qPCR were used to evaluate cell growth, UPR activation and apoptosis in a panel of leukemia cell lines that included AML, ALL and CML in cells exposed to ML291. CRISPR-Cas9 genome editing was used to delete CHOP in K562 (human myeloid leukemia) cells. Deletion was validated by immunoblot analysis and these cells were subjected to the same proliferation and gene analyses described above. The in vivo response to ML291 therapy was evaluated in an established zebrafish xenograft assay (Corkery et al. BJH 2011) in which embryos were xenotransplanted with wild type or CHOP knockdown K562 cells and embryos bathed in ML291. Results: Immunoblot and RT-qPCR analysis revealed an accumulation of proteins and increased gene expression for downstream UPR genes, including CHOP, GRP78/BiP, GADD34 and XBP1 in leukemia cells following ML291 treatment, indicating the activation of the UPR. Increased expression of the apoptotic genes, NOXA, PUMA and DR5 was also observed post-treatment with ML291; and dose response proliferation assays performed after 24 hours revealed IC50 concentrations of 1 - 30µM across cell lines. CHOP deleted K562 cells were protected from cell death when cultured with increasing concentrations of ML291, and were significantly less able to translocate phosphatidylserine across the cell membrane and activate the caspase cascade. When zebrafish embryos xenotransplanted with K562-wild type or -CHOP-null cells were bathed in water containing 5mM ML291 for three days, there was a significant reduction in leukemia cell burden exclusively in theK562 wild type xenografts. Conclusion: Collectively these data indicate that intact PERK/eIF2a/CHOP signaling is required for efficient leukemic cell apoptosis in response to ML291 in vitro and in vivo, and support the hypothesis that small molecule enforcement of the UPR might be a productive therapeutic approach in leukemia. Disclosures No relevant conflicts of interest to declare.

scholarly journals Two Cell Adhesion Molecules, Nectin and Cadherin, Interact through Their Cytoplasmic Domain–Associated Proteins

2000 ◽  
Vol 150 (5) ◽  
pp. 1161-1176 ◽  
Author(s):  
Kouichi Tachibana ◽  
Hiroyuki Nakanishi ◽  
Kenji Mandai ◽  
Kumi Ozaki ◽  
Wataru Ikeda ◽  
...  
Keyword(s):  
Cell Adhesion ◽  
Cell Lines ◽  
Cell Adhesion Molecules ◽  
Cytoplasmic Domain ◽  
Adhesion Sites ◽  
Adhesion System ◽  
Associated Proteins ◽  
Mechanism Of Interaction ◽  
E Cadherin ◽  
Cell Cell

We have found a new cell–cell adhesion system at cadherin-based cell–cell adherens junctions (AJs) consisting of at least nectin and l-afadin. Nectin is a Ca2+-independent homophilic immunoglobulin-like adhesion molecule, and l-afadin is an actin filament-binding protein that connects the cytoplasmic region of nectin to the actin cytoskeleton. Both the trans-interaction of nectin and the interaction of nectin with l-afadin are necessary for their colocalization with E-cadherin and catenins at AJs. Here, we examined the mechanism of interaction between these two cell–cell adhesion systems at AJs by the use of α-catenin–deficient F9 cell lines and cadherin-deficient L cell lines stably expressing their various components. We showed here that nectin and E-cadherin were colocalized through l-afadin and the COOH-terminal half of α-catenin at AJs. Nectin trans-interacted independently of E-cadherin, and the complex of E-cadherin and α- and β-catenins was recruited to nectin-based cell–cell adhesion sites through l-afadin without the trans-interaction of E-cadherin. Our results indicate that nectin and cadherin interact through their cytoplasmic domain–associated proteins and suggest that these two cell–cell adhesion systems cooperatively organize cell–cell AJs.

scholarly journals Translational diffusion of class II major histocompatibility complex molecules is constrained by their cytoplasmic domains.

1989 ◽  
Vol 109 (6) ◽  
pp. 3325-3331 ◽  
Author(s):  
W F Wade ◽  
J H Freed ◽  
M Edidin
Keyword(s):  
Amino Acids ◽  
Plasma Membrane ◽  
Major Histocompatibility Complex ◽  
Cytoplasmic Domain ◽  
Translational Diffusion ◽  
Type I ◽  
Beta Chain ◽  
Wild Type ◽  
Alpha Chain ◽  
Histocompatibility Complex

Site-directed mutagenesis in vitro was used to introduce stop codons in the genomic DNA of the alpha and beta chains of the murine class II major histocompatibility complex antigen, I-Ak. Mutated DNA was transfected into B lymphoma cells that were then selected by neomycin resistance and for their ability to express I-Ak molecules on their plasma membrane. The translational diffusion coefficient (Dlat) of I-Ak molecules composed of a wild-type beta chain paired with an alpha chain missing either 6 or 12 amino acids from the cytoplasmic domain is on the average threefold higher than the Dlat of wild-type I-Ak molecules as measured by fluorescence photobleaching and recovery. The removal of 12 amino acids from the cytoplasmic domain of the beta chain did not change the Dlat value from that of wild-type I-Ak if the truncated beta chain was paired with a wild-type alpha chain. Removing all amino acids of the cytoplasmic domains of both the alpha and beta chains resulted in a 10-fold increase in the Dlat, the highest value for any of the truncated I-Ak molecules tested. These data indicate that the carboxy-terminal six amino acids of the cytoplasmic domain of the alpha chain and the six plasma membrane-proximal amino acids of the beta chain are important in constraining the translational diffusion of I-Ak molecules in the plasma membrane.

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