Natural killer (NK) cells eliminate cancer and virus-infected cells through cytolytic activity. The last step in NK cell cytotoxicity, resulting in exocytosis of granule content, requires fusion of lytic granules with the plasma membrane. Proteins from the SNARE family mediate membrane fusion events in the cell. Here we show that NK cells express all members of the R-SNARE subgroup. Two of these R-SNARE proteins, VAMP4 and VAMP7, co-localize with lytic granules during cytotoxic interactions. However, only VAMP7 associates with perforin-containing granules in non-activated cells, indicating that the two VAMPs have different functions in exocytosis. Using both the tumor NK cell line, YTS, and peripheral NK cells we show that disruption of expression of either VAMP4 or VAMP7 inhibits release of lytic granules and severely impairs NK cell cytotoxic activity. Furthermore, VAMP7 but not VAMP4 is involved in IFNγ secretion in NK cells, indicating that VAMP7 is involved in many fusion processes and thus plays a more general function in NK cell activity than VAMP4.
Keywords: lytic granules, perforin, vesicle fusion, immunological synapse