Intracellular processing and antigenic maturation of measles virus hemagglutinin protein.

The intracellular processing and antigenic maturation of the measles virus (MV) hemagglutinin (H) protein in virus infected cells were probed with murine monoclonal antibodies (Mabs) that reacted with continuous and discontinuous epitopes. The antibodies distinguished between the immature, cotranslational monomeric form of the protein and the mature, dimeric hemagglutinin structure. This was evidenced by testing of immunoreactivity of the Mabs with synthetic peptides, by in vitro synthesized H protein analysis, and by pulse-chase analysis of gel separated monomeric and dimeric forms of the H protein. Time kinetics analysis showed that the protein was synthesized as monomers and most of them were converted into dimers with t1/2 about 30 min. The H protein remained endoglycosidase H (Endo H) sensitive up to 30 min and started to acquire partial resistance to Endo H between 30 and 60 min (t1/2 about 60 min) after synthesis. Oligomerization of the H protein was unaffected in virus infected cells treated with a compound (carbonylcyanide m-chlorophenylhydrazone, CCCP) that blocks transport from the endoplasmic reticulum (ER) to the Golgi complex. These results suggest that the H protein dimerization takes place in the ER before its transport to the medial Golgi complex. The Mabs specific for discontinuous epitopes reacted with the H protein in cells treated with CCCP. Thus conformational antigenic epitope formation appears to take place in the ER.