Cell surface accumulation of overexpressed hamster lysosomal membrane glycoproteins

We cloned and sequenced cDNAs encoding two lysosomal membrane glycoproteins, lgp-A and lgp-B, from Chinese hamster ovary cells. The deduced amino acid sequences of these proteins are similar to those of the other known members of this conserved family (also known as "LAMP" proteins). We used the cDNAs to generate stable lines of hamster lgp-expressing mouse NIH-3T3 cells, rat NRK cells, and monkey CV-1 cells. We also generated hybridomas that secrete antibodies specific for hamster lgp-A and lgp-B, enabling us to distinguish foreign from endogenous lgps in a wider variety of transfected cell lines. One line of mouse NIH-3T3 cells that expresses hamster lgp-B was studied in detail. Whereas most of the hamster lgp-B appeared to be transported to lysosomes in these cells, butyrate-induced overexpression resulted in the accumulation of a significant proportion of the total on the plasma membrane. In addition, overexpression of this foreign lgp-B also resulted in the appearance of the endogenous mouse lgp-A and lgp-B on the plasma membrane. Characterization of this accumulation suggested that it resulted from competition for one or more limited components in the transport pathway(s) to lysosomes. Endocytosis from the plasma membrane appeared to be one step that was saturable.