U. Tatu et al., FOLDING AND OLIGOMERIZATION OF INFLUENZA HEMAGGLUTININ IN THE ER AND THE INTERMEDIATE COMPARTMENT, EMBO journal, 14(7), 1995, pp. 1340-1348
Influenza hemagglutinin (HA) was used to analyze the stepwise folding
and oligomeric assembly of glycoproteins in the early secretory pathwa
y of living cells. In addition to mature trimers, six distinct maturat
ion intermediates were identified. Of these, all the incompletely oxid
ized forms were located in the endoplasmic reticulum (ER) and associat
ed with calnexin, a membrane-bound, lectin-like ER chaperone. Once ful
ly oxidized, the HA dissociated from calnexin as a monomer, which rapi
dly became resistant to dithiothreitol (DTT) reduction. Part of these
extensively folded molecules moved as monomers into the intermediate c
ompartment between the ER and the Golgi complex. Assembly of homotrime
rs occurred without calnexin-involvement within the ER and in the inte
rmediate compartment. When anchor-free HA molecules were analyzed, it
was found that they reach the DTT-resistant monomeric conformation but
fail to trimerize. Taken together, the results provide a definition a
nd intracellular localization of several intermediates in the conforma
tional maturation of HA, including the immediate precursor for trimer
assembly.