Structural requirements for major histocompatibility complex class II invariant chain endocytosis and lysosomal targeting.
The invariant chain (Ii) targets newly synthesized major histocompatibility complex class II complexes to a lysosome-like compartment. Previously, we demonstrated that both the cytoplasmic tail (CT) and transmembrane (TM) domains of Ii were sufficient for this targeting and that the CT contains two di-leucine signals, 3DQRDLI8 and 12EQLPML17 (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 for endocytosis and those required for lysosomal targeting by analyzing Ii-transferrin receptor chimeras in quantitative transport assays. Analysis of the Ii CT signals indicates that although 3DQRDLI8 is necessary and sufficient for endocytosis, either di-leucine signal is sufficient 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 coded 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-type transferrin receptor, lysosomal targeting was dramatically enhanced, whereas endocytosis remained unchanged. Our results therefore demonstrate that the structural requirements for Ii endocytosis and lysosomal targeting are different.
Department of Cell Biology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0005, USA.
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Kang
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Liang
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Parker
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Collawn
D000595:Amino Acid Sequence; D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D000944:Antigens, Differentiation, B-Lymphocyte / Q000737:chemistry*; D002642:Chick Embryo; D004705:Endocytosis / Q000502:physiology*; D000949:Histocompatibility Antigens Class II / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008247:Lysosomes / Q000502:physiology*; D008565:Membrane Proteins / Q000737:chemistry; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D011990:Receptors, Transferrin / Q000737:chemistry; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D013329:Structure-Activity Relationship
D000944:Antigens, Differentiation, B-Lymphocyte; D000949:Histocompatibility Antigens Class II; D008565:Membrane Proteins; D011990:Receptors, Transferrin; D011993:Recombinant Fusion Proteins; C045084:invariant chain; D000643:Ammonium Chloride