Structural analysis of proteolytic products of MHC class II-invariant chain complexes generated in vivo.
The MHC class II alpha beta heterodimer associates with invariant (I) chain in the endoplasmic reticulum and remains associated until the complex reaches a post-Golgi compartment. During early stages of transport, I chain blocks peptide binding to alpha beta dimers. I chain is proteolytically cleaved in a post-Golgi compartment releasing alpha beta dimers that can bind antigenic peptides and transport them to the cell surface. Human B lymphoblastoid cell lines grown in leupeptin, a sulfhydryl protease inhibitor, accumulate a partial proteolytic product of the I chain called leupeptin-induced protein (LIP). LIP remains associated with alpha beta dimers. We find, using chemical cross-linking, sucrose gradient sedimentation, and size exclusion chromatography, that the alpha beta LIP complex retains the nine-subunit structure described for alpha beta I complexes. Unlike the alpha beta I complex, in certain detergents the alpha beta LIP nonamer is unstable and dissociates into trimers containing one alpha, beta, and LIP molecule. This finding emphasizes the reported stoichiometry of the alpha beta I complex as a nine-subunit structure comprised of three alpha beta I trimers. Also, these data indicate that the region(s) of I chain necessary for retaining the nonameric structure lie within the LIP fragment, but that domains to the C-terminus of the LIP cleavage site act to further stabilize the nine chain structure. In addition, alpha beta I complexes containing forms of human I chain encoding the p35/p43 N-terminal cytoplasmic extension responsible for endoplasmic reticulum retention can transport to post-Golgi proteolytic compartments where LIP is formed.
Newcomb
J R
JR
Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06510.
Cresswell
P
P
D000944:Antigens, Differentiation, B-Lymphocyte; D001692:Biological Transport; D002460:Cell Line; D002499:Centrifugation, Density Gradient; D002793:Cholic Acids / Q000494:pharmacology; D002850:Chromatography, Gel; D000949:Histocompatibility Antigens Class II / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology*; D008970:Molecular Weight; D010446:Peptide Fragments / Q000737:chemistry*
D000944:Antigens, Differentiation, B-Lymphocyte; D002793:Cholic Acids; D000949:Histocompatibility Antigens Class II; D007976:Leupeptins; D010446:Peptide Fragments; C045084:invariant chain; C032854:leupeptin; C028213:3-((3-cholamidopropyl)dimethylammonium)-1-propanesulfonate