In NK cells, beta2 integrin binding to ICAM family members is sufficient to promote not only adhesion to target cells, but also the polarization of intracellular lytic granules towards the target. Using unbiased mass spectrometry and bioinformatics analysis of NK cells that had been stimulated through beta2 integrin LFA-1 resulted in the discovery of a complete signaling pathway that controls granule polarization toward the target cell. In addition to a network of 11 proteins tested for a role in granule polarization, there were 12 orphan proteins with no known interactions. The orphan protein with the highest score in the mass spectrometry analysis was cytohesin 4. The four members of the cytohesin family carry a PH domain and a Sec7 domain with guanine exchange factor activity for members of the small GTPase Arf family. While cytohesin 1 plays a role in inside-out signaling for beta2 integrin LFA-1, the function of cytohesin 4 is unknown. Silencing of cytohesin 4 but not cytohesin 1 resulted in defective granule polarization. To our knowledge, this is the first time a function has been attributed to cytohesin 4.
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