scholarly journals Life and Death of Fungal Transporters Under the Challenge of Polarity

2020 ◽  
Author(s):  
Sofia Dimou ◽  
George Diallinas
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Membrane Proteins ◽  
Cell Polarity ◽  
Secretory Pathway ◽  
Fungal Growth ◽  
Present Evidence ◽  
Life And Death ◽  
Early Endosomes ◽  
Stress Signals

Eukaryotic plasma membrane (PM) transporters face critical challenges that are not widely present in prokaryotes. The two most important issues are proper subcellular traffic and targeting to the PM, and regulated endocytosis in response to physiological, developmental or stress signals. Sorting of transporters from their site of synthesis, the Endoplasmic Reticulum (ER), to the PM has been long thought, but not formally shown, to occur via the conventional Golgi-dependent vesicular secretory pathway. Endocytosis of specific eukaryotic transporters has been studied more systematically and shown to involve ubiquitination, internalization, and sorting to early endosomes, followed by turnover in the MVB/lysosomes/vacuole system. In specific cases internalized transporters have been shown to recycle back to the PM. However, the mechanisms of transporter forward trafficking and turnover have been overturned recently through systematic work in the model fungus Aspergillus nidulans. In this review we present evidence that shows that transporter traffic to the PM takes place through Golgi-bypass and transporter endocytosis operates via a mechanism that is distinct from that of recycling membrane cargoes essential for fungal growth. We discuss these findings in relation to adaptation to challenges imposed by cell polarity in fungi as well as in other eukaryotes and provide a rationale why transporters and possibly other housekeeping membrane proteins ‘avoid’ routes of polar trafficking.

scholarly journals Life and Death of Fungal Transporters under the Challenge of Polarity

2020 ◽  
Vol 21 (15) ◽  
pp. 5376
Author(s):  
Sofia Dimou ◽  
George Diallinas
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Cell Polarity ◽  
Secretory Pathway ◽  
Fungal Growth ◽  
Multivesicular Bodies ◽  
Present Evidence ◽  
Life And Death ◽  
Early Endosomes ◽  
Stress Signals

Eukaryotic plasma membrane (PM) transporters face critical challenges that are not widely present in prokaryotes. The two most important issues are proper subcellular traffic and targeting to the PM, and regulated endocytosis in response to physiological, developmental, or stress signals. Sorting of transporters from their site of synthesis, the endoplasmic reticulum (ER), to the PM has been long thought, but not formally shown, to occur via the conventional Golgi-dependent vesicular secretory pathway. Endocytosis of specific eukaryotic transporters has been studied more systematically and shown to involve ubiquitination, internalization, and sorting to early endosomes, followed by turnover in the multivesicular bodies (MVB)/lysosomes/vacuole system. In specific cases, internalized transporters have been shown to recycle back to the PM. However, the mechanisms of transporter forward trafficking and turnover have been overturned recently through systematic work in the model fungus Aspergillus nidulans. In this review, we present evidence that shows that transporter traffic to the PM takes place through Golgi bypass and transporter endocytosis operates via a mechanism that is distinct from that of recycling membrane cargoes essential for fungal growth. We discuss these findings in relation to adaptation to challenges imposed by cell polarity in fungi as well as in other eukaryotes and provide a rationale of why transporters and possibly other housekeeping membrane proteins ‘avoid’ routes of polar trafficking.

scholarly journals Transmembrane domain–dependent partitioning of membrane proteins within the endoplasmic reticulum

2008 ◽  
Vol 181 (1) ◽  
pp. 105-118 ◽  
Author(s):  
Paolo Ronchi ◽  
Sara Colombo ◽  
Maura Francolini ◽  
Nica Borgese
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Membrane Proteins ◽  
Secretory Pathway ◽  
Transmembrane Domain ◽  
Fluorescent Proteins ◽  
Lipid Interactions ◽  
Er Exit Sites ◽  
Physicochemical Features ◽  
Er Membrane

The length and hydrophobicity of the transmembrane domain (TMD) play an important role in the sorting of membrane proteins within the secretory pathway; however, the relative contributions of protein–protein and protein–lipid interactions to this phenomenon are currently not understood. To investigate the mechanism of TMD-dependent sorting, we used the following two C tail–anchored fluorescent proteins (FPs), which differ only in TMD length: FP-17, which is anchored to the endoplasmic reticulum (ER) membrane by 17 uncharged residues, and FP-22, which is driven to the plasma membrane by its 22-residue-long TMD. Before export of FP-22, the two constructs, although freely diffusible, were seen to distribute differently between ER tubules and sheets. Analyses in temperature-blocked cells revealed that FP-17 is excluded from ER exit sites, whereas FP-22 is recruited to them, although it remains freely exchangeable with the surrounding reticulum. Thus, physicochemical features of the TMD influence sorting of membrane proteins both within the ER and at the ER–Golgi boundary by simple receptor-independent mechanisms based on partitioning.

scholarly journals Septin-dependent compartmentalization of the endoplasmic reticulum during yeast polarized growth

2005 ◽  
Vol 169 (6) ◽  
pp. 897-908 ◽  
Author(s):  
Cosima Luedeke ◽  
Stéphanie Buvelot Frei ◽  
Ivo Sbalzarini ◽  
Heinz Schwarz ◽  
Anne Spang ◽  
...  
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Membrane Proteins ◽  
Diffusion Barriers ◽  
Yeast Cells ◽  
Dependent Manner ◽  
Protein Diffusion ◽  
Ultrastructural Studies ◽  
Bud Neck ◽  
Er Membrane

Polarized cells frequently use diffusion barriers to separate plasma membrane domains. It is unknown whether diffusion barriers also compartmentalize intracellular organelles. We used photobleaching techniques to characterize protein diffusion in the yeast endoplasmic reticulum (ER). Although a soluble protein diffused rapidly throughout the ER lumen, diffusion of ER membrane proteins was restricted at the bud neck. Ultrastructural studies and fluorescence microscopy revealed the presence of a ring of smooth ER at the bud neck. This ER domain and the restriction of diffusion for ER membrane proteins through the bud neck depended on septin function. The membrane-associated protein Bud6 localized to the bud neck in a septin-dependent manner and was required to restrict the diffusion of ER membrane proteins. Our results indicate that Bud6 acts downstream of septins to assemble a fence in the ER membrane at the bud neck. Thus, in polarized yeast cells, diffusion barriers compartmentalize the ER and the plasma membrane along parallel lines.

scholarly journals pH-dependent Cargo Sorting from the Golgi

2011 ◽  
Vol 286 (12) ◽  
pp. 10058-10065 ◽  
Author(s):  
Chunjuan Huang ◽  
Amy Chang
Keyword(s):  
Plasma Membrane ◽  
Membrane Proteins ◽  
Atpase Activity ◽  
Secretory Pathway ◽  
Ph Dependence ◽  
Glucose Deprivation ◽  
Ph Homeostasis ◽  
Plasma Membrane Proteins ◽  
Cytosolic Ph ◽  
Cargo Sorting

The vacuolar proton-translocating ATPase (V-ATPase) plays a major role in organelle acidification and works together with other ion transporters to maintain pH homeostasis in eukaryotic cells. We analyzed a requirement for V-ATPase activity in protein trafficking in the yeast secretory pathway. Deficiency of V-ATPase activity caused by subunit deletion or glucose deprivation results in missorting of newly synthesized plasma membrane proteins Pma1 and Can1 directly from the Golgi to the vacuole. Vacuolar mislocalization of Pma1 is dependent on Gga adaptors although no Pma1 ubiquitination was detected. Proper cell surface targeting of Pma1 was rescued in V-ATPase-deficient cells by increasing the pH of the medium, suggesting that missorting is the result of aberrant cytosolic pH. In addition to mislocalization of the plasma membrane proteins, Golgi membrane proteins Kex2 and Vrg4 are also missorted to the vacuole upon loss of V-ATPase activity. Because the missorted cargos have distinct trafficking routes, we suggest a pH dependence for multiple cargo sorting events at the Golgi.

scholarly journals The endosomal transcriptional regulator RNF11 integrates degradation and transport of EGFR

2016 ◽  
Vol 215 (4) ◽  
pp. 543-558 ◽  
Author(s):  
Sandra Scharaw ◽  
Murat Iskar ◽  
Alessandro Ori ◽  
Gaelle Boncompain ◽  
Vibor Laketa ◽  
...  
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Egf Receptor ◽  
Regulatory Mechanism ◽  
Transcriptional Regulator ◽  
Transport Efficiency ◽  
Early Endosomes ◽  
Epidermal Growth ◽  
Partial Degradation ◽  
Stimulation Of

Stimulation of cells with epidermal growth factor (EGF) induces internalization and partial degradation of the EGF receptor (EGFR) by the endo-lysosomal pathway. For continuous cell functioning, EGFR plasma membrane levels are maintained by transporting newly synthesized EGFRs to the cell surface. The regulation of this process is largely unknown. In this study, we find that EGF stimulation specifically increases the transport efficiency of newly synthesized EGFRs from the endoplasmic reticulum to the plasma membrane. This coincides with an up-regulation of the inner coat protein complex II (COPII) components SEC23B, SEC24B, and SEC24D, which we show to be specifically required for EGFR transport. Up-regulation of these COPII components requires the transcriptional regulator RNF11, which localizes to early endosomes and appears additionally in the cell nucleus upon continuous EGF stimulation. Collectively, our work identifies a new regulatory mechanism that integrates the degradation and transport of EGFR in order to maintain its physiological levels at the plasma membrane.

scholarly journals Membrane traffic between secretory compartments is differentially affected during mitosis.

1990 ◽  
Vol 1 (5) ◽  
pp. 415-424 ◽  
Author(s):  
T Kreiner ◽  
H P Moore
Keyword(s):  
Plasma Membrane ◽  
Membrane Proteins ◽  
Secretory Pathway ◽  
Chinese Hamster Ovary Cells ◽  
Chinese Hamster ◽  
Membrane Traffic ◽  
Human Growth ◽  
Secretory Proteins ◽  
Viral Membrane ◽  
Mitotic Cells

Membrane traffic has been shown to be regulated during cell division. In particular, with the use of viral membrane proteins as markers, endoplasmic reticulum (ER)-to-Golgi transport in mitotic cells has been shown to be essentially blocked. However, the effect of mitosis on other steps in the secretory pathway is less clear, because an early block makes examination of following steps difficult. Here, we report studies on the functional characteristics of secretory pathways in mitotic mammalian tissue culture cells by the use of a variety of markers. Chinese hamster ovary cells were transfected with cDNAs encoding secretory proteins. Consistent with earlier results following viral membrane proteins, we found that the overall secretory pathway is nonfunctional in mitotic cells, and a major block to secretion is at the step between ER and Golgi: the overall rate of secretion of human growth hormone is reduced at least 10-fold in mitotic cells, and export of truncated vesicular stomatitis virus G protein from the ER is inhibited to about the same extent, as judged by acquisition of endoglycosidase H resistance. To ascertain the integrity of transport from the trans-Golgi to plasma membrane, we followed the secretion of sulfated glycosaminoglycan (GAG) chains, which are synthesized in the Golgi and thus are not subject to the earlier ER-to-Golgi block. GAG chains are valid markers for the pathway taken by constitutive secretory proteins; both protein secretion and GAG chain secretion are sensitive to treatment with n-ethyl-maleimide and monensin and are blocked at 19 degrees C. We found that the extent of GAG-chain secretion is not altered during mitosis, although the initial rate of secretion is reduced about twofold in mitotic compared with interphase cells. Thus, during mitosis, transport from the trans-Golgi to plasma membrane is much less hindered than ER-to-Golgi traffic. We conclude that transport steps are not affected to the same extent during mitosis.

Peroxisomal membrane proteins are properly targeted to peroxisomes in the absence of COPI- and COPII-mediated vesicular transport

2001 ◽  
Vol 114 (11) ◽  
pp. 2199-2204 ◽  
Author(s):  
Tineke Voorn-Brouwer ◽  
Astrid Kragt ◽  
Henk F. Tabak ◽  
Ben Distel
Keyword(s):  
Endoplasmic Reticulum ◽  
Membrane Proteins ◽  
Secretory Pathway ◽  
Human Fibroblasts ◽  
Vesicle Transport ◽  
Peroxisome Biogenesis ◽  
Peroxisomal Membrane ◽  
Classic Model ◽  
Early Secretory Pathway

The classic model for peroxisome biogenesis states that new peroxisomes arise by the fission of pre-existing ones and that peroxisomal matrix and membrane proteins are recruited directly from the cytosol. Recent studies challenge this model and suggest that some peroxisomal membrane proteins might traffic via the endoplasmic reticulum to peroxisomes. We have studied the trafficking in human fibroblasts of three peroxisomal membrane proteins, Pex2p, Pex3p and Pex16p, all of which have been suggested to transit the endoplasmic reticulum before arriving in peroxisomes. Here, we show that targeting of these peroxisomal membrane proteins is not affected by inhibitors of COPI and COPII that block vesicle transport in the early secretory pathway. Moreover, we have obtained no evidence for the presence of these peroxisomal membrane proteins in compartments other than peroxisomes and demonstrate that COPI and COPII inhibitors do not affect peroxisome morphology or integrity. Together, these data fail to provide any evidence for a role of the endoplasmic reticulum in peroxisome biogenesis.

scholarly journals The unconventional biogenesis of Kv7.1-KCNE1 complexes

2020 ◽  
Vol 6 (14) ◽  
pp. eaay4472 ◽  
Author(s):  
Anna Oliveras ◽  
Clara Serrano-Novillo ◽  
Cristina Moreno ◽  
Alicia de la Cruz ◽  
Carmen Valenzuela ◽  
...  
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Secretory Pathway ◽  
Regulatory Subunit ◽  
Long Qt ◽  
Ongoing Debate ◽  
Subcellular Compartment ◽  
Ongoing Controversy ◽  
The Subject ◽  
Complex Forms

The potassium channel Kv7.1 associates with the KCNE1 regulatory subunit to trigger cardiac IKs currents. Although the Kv7.1/KCNE1 complex has received much attention, the subcellular compartment hosting the assembly is the subject of ongoing debate. Evidence suggests that the complex forms either earlier in the endoplasmic reticulum or directly at the plasma membrane. Kv7.1 and KCNE1 mutations, responsible for long QT syndromes, impair association and traffic, thereby altering IKs currents. We found that Kv7.1 and KCNE1 do not assemble in the first stages of their biogenesis. Data support an unconventional secretory pathway for Kv7.1-KCNE1 that bypasses Golgi. This route targets channels to endoplasmic reticulum–plasma membrane junctions, where Kv7.1-KCNE1 assemble. This mechanism helps to resolve the ongoing controversy about the subcellular compartment hosting the association. Our results also provide new insights into IKs channel localization at endoplasmic reticulum–plasma membrane junctions, highlighting an alternative anterograde trafficking mechanism for oligomeric ion channels.

scholarly journals Density of newly synthesized plasma membrane proteins in intracellular membranes II. Biochemical studies.

1984 ◽  
Vol 98 (6) ◽  
pp. 2142-2147 ◽  
Author(s):  
P Quinn ◽  
G Griffiths ◽  
G Warren
Keyword(s):  
Endoplasmic Reticulum ◽  
Plasma Membrane ◽  
Membrane Proteins ◽  
Membrane Protein ◽  
Semliki Forest Virus ◽  
Companion Paper ◽  
Intracellular Membranes ◽  
Surface Areas ◽  
Quantitative Immunoblotting ◽  
Total Membrane

Using two independent methods, incorporation of radioactive amino-acid and quantitative immunoblotting, we have determined that the rate of synthesis of each of the Semliki Forest virus (SFV) proteins in infected baby hamster kidney (BHK) cells is 1.2 X 10(5) copies/cell/min. Given the absolute surface areas of the endoplasmic reticulum and Golgi complex presented in the companion paper (Griffiths, G., G. Warren, P. Quinn , O. Mathieu - Costello , and A. Hoppeler , 1984, J. Cell Biol. 98:2133-2141), and the approximate time spent in these organelles during their passage to the plasma membrane (Green J., G. Griffiths, D. Louvard , P. Quinn , and G. Warren 1981, J. Mol. Biol. 152:663-698), the mean density of each viral protein in these organelles can be calculated to be 90 and 750 molecules/micron 2 membrane, respectively. In contrast, we have determined that the density of total endogenous integral membrane proteins in these organelles is approximately 30,000 molecules/micron 2 so that the spike proteins constitute only 0.28 and 2.3% of total membrane protein in the endoplasmic reticulum and Golgi, respectively. Quantitative immunoblotting was used to give direct estimates of the concentrations of one of the viral membrane protein precursors (E1) in subcellular fractions; these agreed closely with the calculated values. The data are discussed with respect to the sorting of transported proteins from those endogenous to the intracellular membranes.

scholarly journals Segregation in the Golgi complex precedes export of endolysosomal proteins in distinct transport carriers

2017 ◽  
Vol 216 (12) ◽  
pp. 4141-4151 ◽  
Author(s):  
Yu Chen ◽  
David C. Gershlick ◽  
Sang Yoon Park ◽  
Juan S. Bonifacino
Keyword(s):  
Plasma Membrane ◽  
Membrane Proteins ◽  
Golgi Complex ◽  
Adaptor Proteins ◽  
Transmembrane Domains ◽  
The Other ◽  
Present Evidence ◽  
Sorting Signals ◽  
Vesicular Carriers ◽  
Classical Models

Biosynthetic sorting of newly synthesized transmembrane cargos to endosomes and lysosomes is thought to occur at the TGN through recognition of sorting signals in the cytosolic tails of the cargos by adaptor proteins, leading to cargo packaging into coated vesicles destined for the endolysosomal system. Here we present evidence for a different mechanism in which two sets of endolysosomal proteins undergo early segregation to distinct domains of the Golgi complex by virtue of the proteins’ luminal and transmembrane domains. Proteins in one Golgi domain exit into predominantly vesicular carriers by interaction of sorting signals with adaptor proteins, but proteins in the other domain exit into predominantly tubular carriers shared with plasma membrane proteins, independently of signal–adaptor interactions. These findings demonstrate that sorting of endolysosomal proteins begins at an earlier stage and involves mechanisms that partly differ from those described by classical models.

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