Related Experiment Videos
Evidence for an early heavy chain intermediate in the assembly of H-2Db class I MHC molecules
1University of California San Diego, Department of Biology, La Jolla 92093-0063.
Several recently proposed models for the in vivo biogenesis of class I MHC molecules focus on the retention of empty dimers as a postulated intermediate in the assembly of the complete complexes. The data presented in this study support a slightly different model of class I biogenesis, which includes a precursor population of H-2Db heavy chains (HCs) that is retained in the ER of murine cells prior to its association with beta-2 microglobulin (beta 2m). For this study the intracellular ratios of the subunits that comprise class I molecules have been manipulated to generate a transfected cell line which assembles only very small numbers of unstable H-2Db molecules. Immunoprecipitation experiments with this transfected cell line demonstrated that nascent beta 2m was assembled into complete H-2Db heterotrimers more rapidly than nascent H-2Db HCs by normal murine cells. These data were not consistent with the simultaneous retention of the two associated subunits (HC and beta 2m) in a pool of precursor molecules. However, a previously uncharacterized subset of immature H-2Db HCs, which were not associated with beta 2m, has been detected. These immature HCs exhibited several characteristics of a precursor to complete class I molecules and required a supply of endogenously synthesized peptides for their normal processing in vivo.
Several recently proposed models for the in vivo biogenesis of class I MHC molecules focus on the retention of empty dimers as a postulated intermediate in the assembly of the complete complexes. The data presented in this study support a slightly different model of class I biogenesis, which includes a precursor population of H-2Db heavy chains (HCs) that is retained in the ER of murine cells prior to its association with beta-2 microglobulin (beta 2m). For this study the intracellular ratios of the subunits that comprise class I molecules have been manipulated to generate a transfected cell line which assembles only very small numbers of unstable H-2Db molecules. Immunoprecipitation experiments with this transfected cell line demonstrated that nascent beta 2m was assembled into complete H-2Db heterotrimers more rapidly than nascent H-2Db HCs by normal murine cells. These data were not consistent with the simultaneous retention of the two associated subunits (HC and beta 2m) in a pool of precursor molecules. However, a previously uncharacterized subset of immature H-2Db HCs, which were not associated with beta 2m, has been detected. These immature HCs exhibited several characteristics of a precursor to complete class I molecules and required a supply of endogenously synthesized peptides for their normal processing in vivo.