IDENTIFICATION OF GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE AS A CA2-DEPENDENT FUSOGEN IN HUMAN NEUTROPHIL CYTOSOL()

The membrane fusion events observed during neutrophil degranulation ar e important aspects of the immunoregulatory system. In an attempt to u nderstand the regulation of granule-plasma membrane fusion, we have ch aracterizing human neutrophil cytosol for fusion activity, finding tha t 50% of the fusogenic activity could be attributed to members of the annexin family of proteins. The major non-annexin fusion activity (25% of the total cytosolic activity) was enriched by ion exchange chromat ography after depletion of annexins by Ca2+-dependent phospholipid aff inity chromatography. The fusion activity co-purified with a 10,14-kDa dimer identified as leukocyte L1 (which was non-fusogenic), along wit h an approximately 36-kDa protein. This protein was identified as glyc eraldehyde-3-phosphate dehydrogenase (GAPDH) by amino-terminal sequenc ing, and the fusion activity was verified using commercially available GAPDH. GAPDH may play an important role in degranulation because it i s as potent as annexin I on a mass basis and may constitute up to 25% of the total cytosolic fusion activity of the neutrophil.