Specific heterodimer formation is a prerequisite for uroplakins to exit from the endoplasmic reticulum.
Much of the lower urinary tract, including the bladder, is lined by a stratified urothelium forming a highly differentiated, superficial umbrella cell layer. The apical plasma membrane as well as abundant cytoplasmic fusiform vesicles of the umbrella cells is covered by two-dimensional crystals that are formed by four membrane proteins named uroplakins (UPs) Ia, Ib, II, and III. UPs are synthesized on membrane-bound polysomes, and after several co- and posttranslational modifications they assemble into planar crystals in a post-Golgi vesicular compartment. Distension of the bladder may cause fusiform vesicles to fuse with the apical plasma membrane. We have investigated the early stages of uroplakin assembly by expressing the four uroplakins in 293T cells. Transfection experiments showed that, when expressed individually, only UPIb can exit from the endoplasmic reticulum (ER) and move to the plasma membrane, whereas UPII and UPIII reach the plasma membrane only when they form heterodimeric complexes with UPIa and UPIb, respectively. Heterodimer formation in the ER was confirmed by pulse-chase experiment followed by coimmunoprecipitation. Our results indicate that the initial building blocks for the assembly of crystalline uroplakin plaques are heterodimeric uroplakin complexes that form in the ER.
Tu
Liyu
L
Department of Cell Biology, New York University, School of Medicine, New York, New York 10016, USA.
Sun
Tung-Tien
TT
Kreibich
Gert
G
D000818:Animals; D015153:Blotting, Western; D002417:Cattle; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006026:Glycoside Hydrolases / Q000378:metabolism; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D011132:Polyribosomes / Q000378:metabolism; D011233:Precipitin Tests; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D013997:Time Factors; D014162:Transfection; D060170:Uroplakin II; D060171:Uroplakin III; D060167:Uroplakin Ia; D060169:Uroplakin Ib
D018076:DNA, Complementary; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; C086451:UPK1A protein, human; C086452:UPK1B protein, human; C554658:UPK2 protein, human; D060170:Uroplakin II; D060171:Uroplakin III; D060167:Uroplakin Ia; D060169:Uroplakin Ib; D006026:Glycoside Hydrolases