title: The Cool-2/alpha-Pix protein mediates a Cdc42-Rac signaling cascade.
abstract: BACKGROUND
  Cloned-out of library-2 (Cool-2)/PAK-interactive exchange factor (alpha-Pix) was identified through its ability to bind the Cdc42/Rac target p21-activated kinase (PAK) and has been implicated in certain forms of X-linked mental retardation as well as in growth factor- and chemoattractant-coupled signaling pathways. We recently found that the dimeric form of Cool-2 is a specific guanine nucleotide exchange factor (GEF) for Rac, whereas monomeric Cool-2 is a GEF for Cdc42 as well as Rac. However, unlike many GEFs, Cool-2 binds to activated forms of Cdc42 and Rac. Thus, we have investigated the functional consequences of these interactions.
  
  
  RESULTS
  We show that the binding of activated Cdc42 to the Cool-2 dimer markedly enhances its ability to associate with GDP bound Rac1, resulting in a significant activation of Rac-GEF activity. While the Rac-specific GEF activity of Cool-2 is mediated through the Dbl homology (DH) domain from one monomer and the Pleckstrin homology domain from the other, activated Cdc42 interacts with the DH domain, most likely opposite the DH domain binding site for GDP bound Rac. Activated Rac also binds to Cool-2; however, it strongly inhibits the GEF activity of dimeric Cool-2.
  
  
  CONCLUSIONS
  We provide evidence for novel mechanisms of allosteric regulation of the Rac-GEF activity of the Cool-2 dimer, involving stimulatory effects by Cdc42 and feedback inhibition by Rac. These findings demonstrate that by serving as a target for GTP bound Cdc42 and a GEF for Rac, Cool-2 mediates a GTPase cascade where the activation of Cdc42 is translated into the activation of Rac.
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Baird
authors.forename: Qiyu
authors.initials: Q
authors.lastname: Feng
authors.forename: Richard A
authors.initials: RA
authors.lastname: Cerione
mesh_terms: D000818:Animals; D019556:COS Cells; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002522:Chlorocebus aethiops; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D010957:Plasmids / Q000235:genetics; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D064067:Rho Guanine Nucleotide Exchange Factors; D015398:Signal Transduction; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism*; D054462:p21-Activated Kinases; D020744:rac GTP-Binding Proteins / Q000378:metabolism*
chemical_list: D018797:Cell Cycle Proteins; D020662:Guanine Nucleotide Exchange Factors; D064067:Rho Guanine Nucleotide Exchange Factors; D017346:Protein Serine-Threonine Kinases; D054462:p21-Activated Kinases; D020764:cdc42 GTP-Binding Protein; D020744:rac GTP-Binding Proteins