Focal Adhesion Turnover
In The Journal of Cell Biology Anjana Nayal et al. published an article entitled Paxillin phosphorylation at Ser273 localizes a GIT1–PIX–PAK complex and regulates adhesion and protrusion dynamics. In this paper they explore how interactions between Paxillin, GIT1, PAK, and PIX – proteins already identified as regulators of focal adhesion turnover are regulated.
As all good cell biologists do, they immediately turned to phosphorylation and promptly identify serine 273 of paxillin as being phosphorylated. They go on to show that PAK catalyzes this phosphorylation and that GIT binds preferentially to phosphorylated Paxillin. Additionally an S273D phosphomimic Paxillin increases cell migration, membrane extension, and turnover of membrane protrusions.
The authors then notice that “small” focal adhesions that contain zyxin and vinculin are prevalent approximately 1 µm behind the band of actin at the leading edge in the cells expressing S273D paxillin. Importantly these turnover in less than 1 second and are not present in cells expressing S273A paxillin. GIT1 is also found in the small focal adhesions and siRNA against GIT1 results in a loss of the small focal adhesions.
Among the most important findings of the paper is that PAK acts both upstream and downstream of paxillin. This conclusion comes from the combined findings that PAK phosphorylates paxillin (upstream function) and that dominant negative PAK blocks the ability of S273K paxillin to induce small focal adhesions (downstream function). This downstream function is shown to be dependent on PAK interacting with GIT1 and PIX.
Unfortunately, there is no final figure to present their final model. It could have been particularly useful as this is a data rich paper much of which I have not discussed here. However the take-home message is that PAK activates (phosphorylates) Paxillin which in turn recruits GIT1 and Pix to the focal adhesion (for focal adhesion disassembly) and this complex in turn recruits PAK for downstream functions/reactivation of the pathway.