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We have characterized the biosynthesis and intracellular transport of a membrane glycoprotein, designated plgp57, which is found predominantly in the prelysosome compartment (PLC) of Madin-Darby bovine kidney cells. In pulse-chase experiments, plgp57 was found to be initially synthesized as a 35 kDa precursor which was modified to yield a diffuse approximately 57 kDa mature form. Digestion with endoglycosidase H (endo H) demonstrated that the 35 kDa precursor contained three endo H-sensitive, high mannose-type oligosaccharides which became modified to endo H-resistant, complex-type sugars on the approximately 57 kDa mature form. Labelling cells in the presence of tunicamycin and treatment of the 35 kDa precursor with endo H revealed that plgp57 has a core protein of 24 kDa to which three N-asparagine-liked oligosaccharides are attached. Other experiments indicated that plgp57 could be differentially glycosylated on a common 24 kDa core protein in different cell types. The half-lives of the glycosylated and non-glycosylated forms of plgp57 were approximately 18 and approximately 13 h, respectively. Glycosylated and non-glycosylated plgp57 exhibited similar steady-state intracellular distributions, indicating that targeting of plgp57 to the PLC does not require carbohydrate address markers. Pulse-labeling of cells followed by organelle fractionation at various chase times revealed a t1/2 = approximately 1 h for the transit of newly synthesized plgp57 to the PLC. Finally, amino terminal sequencing of plgp57 revealed the similarity of this protein to the CD63/ME491 family of membrane glycoproteins.

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