STRUCTURAL REQUIREMENTS FOR MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-IIINVARIANT CHAIN ENDOCYTOSIS AND LYSOSOMAL TARGETING

The invariant chain (Ii) targets newly synthesized major histocompatib ility complex class II complexes to a lysosome-like compartment. Previ ously, we demonstrated that both the cytoplasmic tail (CT) and transme mbrane (TM) domains of Ii were sufficient for this targeting and that the CT contains two di-leucine signals, (3)DQRDLI(8) and (12)EQLPML(17 ) (Odorizzi, C, G., Trowbridge, I. S,, Xue, L., Hopkins, C. R., Davis, C. D., and Collawn, J. F, (1994) J. Cell Biol. 126, 317-330). In the present study, we examined the relationship between signals required f or endocytosis and those required for lysosomal targeting by analyzing Ii-transferrin receptor chimeras in quantitative transport assays. An alysis of the Ii CT signals indicates that although 3DQRDLI8 is necess ary and sufficient for endocytosis, either dileucine signal is suffici ent for lysosomal targeting. Deletions between the two signals reduced endocytosis without affecting lysosomal targeting. Transplantation of the DQRDLI sequence in place of the EQLPML signal produced a chimera that trafficked normally, suggesting that this di-leucine sequence cod ed for an independent structural motif. Structure-function analysis of the Ii TM region showed that when Ii TM residues 11-19 and 20-29 were individually substituted for the corresponding regions in the wild-ty pe transferrin receptor, lysosomal targeting was dramatically enhanced , whereas endocytosis remained unchanged. Our results therefore demons trate that the structural requirements for Ii endocytosis and lysosoma l targeting are different.