Annexin II enhances cytomegalovirus binding and fusion to phospholipid membranes

A number of studies have suggested that the anionic phospholipid (anPL)-bin ding protein annexin II may play a role in cytomegalovirus (CMV) infection. Since annexin II has been shown to mediate aggregation and fusion of certa in membranes, we investigated whether these properties could be exploited b y CMV directly. The experiments showed that purified annexin II, but not th e homologous protein annexin V (AnV), can mediate the binding of S-35-CMV ( strain AD169) to anPL-coated microtiter wells. This association required Ca 2+, could be titrated by varying either annexin II (apparent K-d = 4 x 10(- 8) M) or S-35-CMV, was inhibited by unlabeled CMV, and was observed for the heterotetrameric or monomeric form of annexin II. In experiments utilizing the fluorescence dequenching of octadecyl rhodamine incorporated into the CMV envelope, annexin II was furthermore :Found to enhance the rate of viru s-anPL vesicle fusion. The observed fusion was dependent on the concentrati on of annexin II, Ca2+, and anPL and was mediated principally by the hetero tetramer. Interestingly, AnV was observed to inhibit the effects of annexin II on CMV fusion but not binding to anPL, which indicates that annexin II enhances these processes by distinct mechanisms. The results presented here provide the first direct evidence that annexin II has the capacity to brid ge CMV to a phospholipid membrane and to enhance virus-membrane fusion. The se observations furthermore suggest that AnV may regulate the fusogenic fun ction of annexin II.