CELL-SURFACE ACCUMULATION OF OVEREXPRESSED HAMSTER LYSOSOMAL MEMBRANE-GLYCOPROTEINS

We cloned and sequenced cDNAs encoding two lysosomal membrane glycopro teins, Igp-A and Igp-B, from Chinese hamster ovary cells. The deduced amino acid sequences of these proteins are similar to those of the oth er known members of this conserved family (also known as ''LAMP'' prot eins). We used the cDNAs to generate stable lines of hamster Igp-expre ssing mouse NIH-3T3 cells, rat NRK cells, and monkey CV-1 cells. We al so generated hybridomas that secrete antibodies specific for hamster I gp-A and Igp-B, enabling us to distinguish foreign from endogenous Igp s in a wider variety of transfected cell lines. One line of mouse NIH- 3T3 cells that expresses hamster Igp-B was studied in detail. Whereas most of the hamster Igp-B appeared to be transported to lysosomes in t hese cells, butyrate-induced overexpression resulted in the accumulati on of a significant proportion of the total on the plasma membrane. In addition, overexpression of this foreign Igp-B also resulted in the a ppearance of the endogenous mouse Igp-A and Igp-B on the plasma membra ne. Characterization of this accumulation suggested that it resulted f rom competition for one or more limited components in the transport pa thway(s) to lysosomes. Endocytosis from the plasma membrane appeared t o be one step that was saturable.