Assembly of urothelial plaques: tetraspanin function in membrane protein trafficking.
The apical surface of mammalian urothelium is covered by 16-nm protein particles packed hexagonally to form 2D crystals of asymmetric unit membranes (AUM) that contribute to the remarkable permeability barrier function of the urinary bladder. We have shown previously that bovine AUMs contain four major integral membrane proteins, i.e., uroplakins Ia, Ib, II, and IIIa, and that UPIa and Ib (both tetraspanins) form heterodimers with UPII and IIIa, respectively. Using a panel of antibodies recognizing different conformational states of uroplakins, we demonstrate that the UPIa-dependent, furin-mediated cleavage of the prosequence of UPII leads to global conformational changes in mature UPII and that UPIb also induces conformational changes in its partner UPIIIa. We further demonstrate that tetraspanins CD9, CD81, and CD82 can stabilize their partner protein CD4. These results indicate that tetraspanin uroplakins, and some other tetraspanin proteins, can induce conformational changes leading to the ER-exit, stabilization, and cell surface expression of their associated, single-transmembrane-domained partner proteins and thus can function as "maturation-facilitators." We propose a model of AUM assembly in which conformational changes in integral membrane proteins induced by uroplakin interactions, differentiation-dependent glycosylation, and the removal of the prosequence of UPII play roles in regulating the assembly of uroplakins to form AUM.
Hu
Chih-Chi Andrew
CC
Epithelial Biology Unit, The Ronald O. Perelman Department of Dermatology, New York University School of Medicine, New York, NY 10016, USA.
Liang
Feng-Xia
FX
Zhou
Ge
G
Tu
Liyu
L
Tang
Chih-Hang Anthony
CH
Zhou
Jessica
J
Kreibich
Gert
G
Sun
Tung-Tien
TT
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal; D002417:Cattle; D002454:Cell Differentiation / Q000502:physiology; D002478:Cells, Cultured; D019281:Dimerization; D006031:Glycosylation; D008562:Membrane Glycoproteins / Q000378:metabolism; D008565:Membrane Proteins / Q000276:immunology / Q000378:metabolism* / Q000502:physiology*; D008969:Molecular Sequence Data; D011498:Protein Precursors / Q000276:immunology / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D060170:Uroplakin II; D060171:Uroplakin III; D019459:Urothelium / Q000166:cytology / Q000378:metabolism* / Q000502:physiology
D000911:Antibodies, Monoclonal; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D011498:Protein Precursors; D060170:Uroplakin II; D060171:Uroplakin III