Cool/Pix proteins comprise a family of guanine nucleotide exchange factors (GEFs) localized to focal adhesions. The family consists of two isoforms, cool2/αpix and cool1/βPix, the latter having two splice variants that vary in their carboxy termini (1). Cool1/βPix, like other GEFs, has a DH (Dbl homology) domain, which allows binding of small GTPases and GDP/GTP exchange, and a PH (Pleckstrin homology) domain, which is important in regulating subcellular localization. Cool1/βPix also has an SH3 domain, which binds to the PAK kinase, a downstream effector of cdc42 and Rac (3,4). Phosphorylation of cool1/βPix by PAK2 downstream of MAPK signaling alters the localization of a complex containing PAK2 and cool-1/βPix, regulating formation of growth cones in response to growth factors (4). Growth factor induced activation of Rac1 via cool1/βPix was later shown to occur independently of subcellular localization (5). Endothelin-1 stimulation of mesangial cells stimulates the protein kinase A (PKA) pathway, resulting in translocation of cool-1/βPix and activation of cdc42 (6).
