ASSEMBLY AND PEPTIDE BINDING OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II HETERODIMERS IN AN IN-VITRO TRANSLATION SYSTEM

In vitro transcription/translation of HLA-DR1 cDNAs in the presence of microsomal membranes was used to study the association of major histo compatibility complex class II molecules with peptide and invariant ch ain (Ii) in the endoplasmic reticulum (ER). HLA-DR alpha and HLA-DR be ta submits assembled into SDS-unstable heterodimers in the absence of exogenous peptide. The inclusion of synthetic peptides during the alph a/beta assembly process promoted their conversion to SDS-resistant het erodimers. Addition of Ii RNA during the translation of HLA-DR alpha a nd HLA-DR beta RNAs resulted in the formation of alpha/beta/Ii complex es. Peptide binding by class II molecules was detected even when exces s Ii was present during alpha/beta assembly. These findings indicate t hat peptides can bind alpha/beta heterodimers in the ER microenvironme nt and suggest that peptides derived from cytosolic proteins that are presented by class II molecules at the cell surface may have bound to HLA-DR in the ER.