title: BAP31 and its caspase cleavage product regulate cell surface expression of tetraspanins and integrin-mediated cell survival.
abstract: BAP31, a resident integral protein of the endoplasmic reticulum membrane, regulates the export of other integral membrane proteins to the downstream secretory pathway. Here we show that cell surface expression of the tetraspanins CD9 and CD81 is compromised in mouse cells from which the Bap31 gene has been deleted. CD9 and CD81 facilitate the function of multiprotein complexes at the plasma membrane, including integrins. Of note, BAP31 does not appear to influence the egress of alpha5beta1 or alpha(v)beta3 integrins to the cell surface, but in Bap31-null mouse cells, these integrins are not able to maintain cellular adhesion to the extracellular matrix in the presence of reduced serum. Consequently, Bap31-null cells are sensitive to serum starvation-induced apoptosis. Reconstitution of wild-type BAP31 into these Bap31-null cells restores integrin-mediated cell attachment and cell survival after serum stress, whereas interference with the functions of CD9, alpha5beta1, or alpha(v)beta3 by antagonizing antibodies makes BAP31 cells act similar to Bap31-null cells in these respects. Finally, in human KB epithelial cells protected from apoptosis by BCL-2, the caspase-8 cleavage product, p20 BAP31, inhibits egress of tetraspanin and integrin-mediated cell attachment. Thus, p20 BAP31 can operate upstream of BCL-2 in living cells to influence cell surface properties due to its effects on protein egress from the endoplasmic reticulum.
authors.affiliation: Department of Biochemistry, McIntyre Medical Sciences Building, McGill University, Montreal, Quebec H3G 1Y6, Canada.
authors.forename: Marina
authors.initials: M
authors.lastname: Stojanovic
authors.forename: Marc
authors.initials: M
authors.lastname: Germain
authors.forename: Mai
authors.initials: M
authors.lastname: Nguyen
authors.forename: Gordon C
authors.initials: GC
authors.lastname: Shore
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000096:biosynthesis; D017209:Apoptosis; D053181:Caspase 8; D020169:Caspases / Q000378:metabolism; D002448:Cell Adhesion; D002462:Cell Membrane / Q000378:metabolism; D002469:Cell Separation; D002470:Cell Survival; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004847:Epithelial Cells / Q000166:cytology; D005109:Extracellular Matrix / Q000378:metabolism; D005353:Fibronectins / Q000378:metabolism; D005434:Flow Cytometry; D017353:Gene Deletion; D039081:Integrin alpha5beta1 / Q000378:metabolism; D039302:Integrin alphaVbeta3 / Q000378:metabolism; D016023:Integrins / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000096:biosynthesis; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000737:chemistry / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D008856:Microscopy, Fluorescence; D017434:Protein Structure, Tertiary; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D060225:Tetraspanin 28; D060245:Tetraspanin 29; D013997:Time Factors
chemical_list: D015703:Antigens, CD; C489642:Bcap31 protein, mouse; C554798:CD81 protein, human; C554869:CD9 protein, human; C554799:Cd81 protein, mouse; C554870:Cd9 protein, mouse; D005353:Fibronectins; D039081:Integrin alpha5beta1; D039302:Integrin alphaVbeta3; D016023:Integrins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D060225:Tetraspanin 28; D060245:Tetraspanin 29; C505530:CASP8 protein, human; C505526:Casp8 protein, mouse; D053181:Caspase 8; D020169:Caspases