Folding, trimerization, and transport are sequential events in the biogenesis of influenza virus hemagglutinin

Results from several systems indicate that correct protein folding and subunit assembly correlate with the transport of membrane and secretory proteins from the endoplasmic reticulum (ER) to the Golgi complex. Because the site of oligomer assembly and its precise relationship to intracellular transport remain unclear, we have studied in detail the folding and trimerization of the influenza virus hemagglutinin (HA0) relative to its transport from ER to Golgi. Trimerization and transport were analyzed using several different methods, including transport inhibitors, temperature blocks, semi-intact cells, in vitro protein translocation, and immunocytochemistry. Taken together, the results clearly demonstrate that trimerization occurs at a point prior to exit from the ER. Before assembly, HA0 monomers were extensively folded and possessed intramolecular disulfide bonds, but monomers were not transported to the cis Golgi compartment. Thus, hemagglutinin progresses through at least two intermediate states before transport to the Golgi: highly folded monomers and trimers that have not yet left the ER.