Finding a Target or Settling Down with One

Abstract

Natural killer (NK) cells navigate to transformed or virus-infected cells and bind to them through integrins and NK receptors to form a "lytic synapse." Both navigation and formation of the lytic synapse depend on the actin cytoskeleton, leading Butler et al . to investigate the role of HS1 (a homolog of the actin-binding protein cortactin) in NK cell-mediated cell cytolysis. When NK cells (or a leukemia-derived NK cell line) were exposed to target cells, or beads coated with the β2 integrin LFA-1 ligand ICAM-1 and the NK receptor ligand ULBP, HS1 localized to the contact site and became tyrosine phosphorylated. Moreover, HS1 knockdown with short hairpin RNA decreased cytolysis of target cells. Experiments in which HS1 was knocked down and cells were transfected with HS1 mutants in which one or both of two tyrosine residues were substituted with phenylalanine (Y378F, Y397F, and Y378FY397F) implicated HS1 phosphorylation in NK cell cytolytic activity and revealed distinct roles for phosphorylation of these two residues. Adhesion to ICAM-1 stimulated HS1 phosphorylation on Tyr 397 ; further, Tyr 397 was required for cell adhesion to ICAM-1. Tyr 397 was required for chemokine-dependent conversion--through "inside-out" signaling--of LFA-1 to a high-affinity state and for signals downstream of LFA-1 involved in recruitment of actin, LFA-1, the actin regulator WASp, and the guanine nucleotide exchange factor Vav1 to the lytic synapse. Although HS1 Tyr 397 was not required for recruitment of the adaptor DAP10 to the NKG2D receptor, it was implicated in downstream signaling. In contrast, phosphorylation of HS1 Tyr 378 was required for chemotaxis. Thus, HS1 appears to be critical to NK cell chemotaxis, formation of the lytic synapse, and cytolysis, with distinct functions depending on phosphorylation of Tyr 397 and Tyr 378 . Beemiller and Krummel provide thoughtful commentary, noting that HS1 may act as a switch to enable NK cells to convert from a migratory mode to one in which they form a stable contact with a target cell. B. Butler, D. H. Kastendieck, J. A. Cooper, Differently phosphorylated forms of the cortactin homolog HS1 mediate distinct functions in natural killer cells. Nat. Immunol. 9 , 887-897 (2008). [PubMed] P. Beemiller, M. F. Krummel, Distinct functions for HS1 in chemosensory versus adhesive signaling. Nat. Immunol. 9 , 833-834 (2008). [PubMed]