ANALYSIS OF THE STRUCTURAL REQUIREMENTS FOR LYSOSOMAL MEMBRANE TARGETING USING TRANSFERRIN RECEPTOR CHIMERAS

The sorting of membrane proteins to the lysosome requires tyrosine-or dileucine-based targeting signals. Recycling receptors have similar si gnals, yet these proteins seldom enter the latter stages of the endocy tic pathway. To determine how lysosomal and internalization signals di ffer, we prepared chimeric molecules consisting of the cytoplasmic tai ls of CDS gamma-chain, lysosomal acid phosphatase, and lysosomal-assoc iated membrane glycoprotein-l, each fused to the transmembrane and ext racellular domains of the transferrin receptor (TR), Each chimera was expressed on the cell surface and rapidly internalized. Metabolic puls e-chase experiments showed that the CD3 gamma-chain and lysosomal acid phosphatase chimeras, unlike the lysosomal-associated membrane glycop rotein chimera, were rapidly degraded in a post-Gels compartment follo wing normal glycosylation. Transplantation of signals from CD3 gamma-c hain and lysosomal acid phosphatase into the TR cytoplasmic tail in pl ace of the native signal, (YTRF23)-T-20, indicated that each signal wa s sufficient to promote endocytosis but not lysosomal targeting of the resulting mutant. Transplantation of two CD3 signals at specific site s in the TR cytoplasmic tail or a single tyrosine-based signal in a tr uncated TR tail, however, was sufficient to promote lysosomal targetin g. Our results therefore suggest that the relative position of the sig nal within the cytoplasmic tail is a critical feature that distinguish es lysosomal targeting signals from internalization signals.