Recently, the Horwitz lab has another paper out, this time in the Journal of Cell Biology on adhesion turnover dynamics in cell migration – Paxillin phosphorylation at Ser273 localizes a GIT1–PIX–PAK complex and regulates adhesion and protrusion dynamics.
Nayal et al. seem to make the current understanding of focal adhesion dynamics much, much more complicated with this paper – the critical part of which is the discovery that p21-activated kinase (PAK) phosphorylates paxillin at serine273, and that this “PAK-dependent paxillin-S273 phosphorylation functions in a positive-feedback loop, as active PAK, active Rac, and myosin II activity are all downstream effectors of this turover pathway. This is a complicated set of protein interactions that we’re talking about here, and Nayal et al. are correct in saying that “Although several regulators of adhesion turnover, including paxillin, G protein-coupled receptor kinase-interacting protein 1 (GIT1), FAK, Src, and PAK are known, how these molecules act together to regulate adhesion turnover is not clear.
For starters, a long series of papers collectively suggest that paxillin targets GIT1 to the leading edge and adhesions, but that this interaction promoted focal adhesion, and that GIT1 regulates protrusion of the leading edge, “raising the possibility that the paxillin-GIT1 interaction may regulate and thus link adhesion turnover and protrusion formation.”
GIT1, in turn, binds to the Rac-GEF PAK-interactive exchange factor (PIX), which in turn binds to the Rac effector PAK, forming a trimolecular GIT1-PIX-PAK signaling complex. This complex has been implicated, in a series of yet more studies, to adhesion formations in several cell types, as well as neuronal and immunological synapse formation. (Figure 4)
Nayal et al. continues:
It is tempting to speculate that the pax–GIT1–PIX–PAK module localizes Rac activity near the leading edge through the joint presence of PIX and PAK. This is consistent with previous studies that show Rac localization near the leading edge and our own observation that Rac activation and cycling is required for rapid adhesion turnover. Genetic studies in Drosophila melanogaster also implicate a positive role for paxillin in the regulation of Rac activity. Finally, recent evidence shows that the Rac-dependent spatial localization of protrusive activity is mediated by active PAK through the recruitment of PIX.
Thus, the overall conclusion is that Ser273 phosphorylation of paxillin, and subsequent formation of the pax–GIT1–PIX–PAK module, links adhesion turnover, protrusion formation, and possibly an intracellular directional cue for further biochemical dynamics at select adhesions. Strikingly, this offers a potential explanation for the identified class of small adhesions residing near the leading edge, which turnover in 20-30s, and resemble paxillin-pS273 localization, in highly motile cells. (Figure 8)
- Paxillin phosphorylation at Ser273 localizes a GIT1-PIX-PAK complex and regulates adhesion and protrusion dynamics. Nayal A, Webb DJ, Brown CM, Schaefer EM, Vicente-Manzanares M, Horwitz AR. J Cell Biol. 2006 May 22; 173(4):587-9. Pubmed