Related Experiment Videos
Structural analysis of proteolytic products of MHC class II-invariant chain complexes generated in vivo
1Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06510.
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.
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.