Location and character of the cellular enzyme that cleaves the hemagglutinin of a virulent avian influenza virus.

H.-D. Klenk, W. Garten, and R. Rott (1984, EMBO J. 3, 2911-2915) have reported that hemagglutinin (HA) cleavage of virulent avian influenza viruses occurs in later steps of its intracellular transport and that the cleavage enzyme is calcium dependent and has a neutral pH optimum. The precise intracellular location of the HA cleavage, however, has never been established. Furthermore, because Klenk et al. used the whole cell lysate to examine the cleavage activity and the amino acid sequencing of the cleaved product was not done, the identity of the cleavage enzyme remains to be established. We therefore attempted to systematically characterize the HA cleavage of the virulent avian virus A/tern/South Africa/61 (H5N3). Using an inhibitor of glycoprotein transport (Brefeldin A) and temporal markers of glycoprotein processing, we found that the endoprotease responsible for the HA cleavage acts after the acquisition of endo-N-acetylglucosaminidase H resistance but before the addition of galactose to the molecule, and thus is located in the medial and/or trans Golgi. This observation was directly confirmed by in vitro experiments using rat liver subcellular membrane fractions containing Golgi complex. A fraction rich in galactosyltransferase (a trans Golgi marker) demonstrated the highest HA cleavage activity. The endoprotease in this fraction cleaved only the HA of the virulent avian influenza virus but not that of an avirulent virus. Through amino-terminal sequencing of the HA2 produced by digestion with the endoprotease in the rat Golgi fraction, we established that HA cleavage by the protease occurs at the authentic site. Further studies using the rat Golgi fraction showed that the HA cleavage enzyme is calcium dependent and has a low pH (6.0) optimum. Thus, the pH optimum of the enzyme in the Golgi fraction differs from that in whole cell lysate reported previously.