Cell-free synthesis and processing of the heavy and light chains of HLA-DR antigens.

Antisera have been prepared against the separated glycoprotein chains (p29 and p34) of HLA-DR antigens (p29,34) isolated from membranes of the B lymphoblastoid cell line JY (DRw4,6). These antisera (anti-p29 and anti-p34) were characterized by immunoprecipitation of in vivo labeled, detergent-solubilized extracts of JY cells grown in the presence and absence of tunicamycin, an inhibitor of N-linked glycosylation. Anti-p29 and anti-p34 specifically immunoprecipitated the precursors to p29 and p34, respectively, from the cell-free translation products of a rabbit reticulocyte lysate system supplemented with polyadenylic acid-containing messenger RNA (mRNA) isolated from JY cells. These precursors, pre-p34 and two pre-p29s, appeared to be 1,500-3,000 daltons larger than their counterparts from tunicamycin-treated cells and presumably were synthesized with N-terminal extensions (signal sequences). Processing of pre-p29 and pre-p34 occurred during cell-free translation in the presence of dog pancreatic microsomes, which resulted in cleavage of the polypeptide, addition of glycan moieties, and segregation of the two chains in the cisternal portion of the microsome. the precursors of p29 and p34 were not appreciably different in isoelectric points on two-dimensional gels from p26 and p28, the light and heavy chains from antigens arise from separate mRNA, and not as a single polypeptide, which is endoproteolytically cleaved.