Dissecting Molecular Interactions Between FAK and Paxillin

February 6, 2008 | By Dan In Journal Club |

In keeping with this week’s trend of just science blogging on FAK, let’s take a look at another critical protein-protein interaction - this time with the scaffolding protein Paxillin. Specifically, how do FAK and Paxillin interact and why?

Conveniently, there’s a recent paper by Danielle Scheswohl et al., from the Schaller lab: Multiple paxillin binding sites regulate FAK function. The motivation to the paper can be found in the abstract: “Recent structural analyses have revealed two paxillin-binding sites in the [focal adhesion targeting, or FAT] domain of FAK. To define the role of paxillin binding to each site on FAK, point mutations have been engineered to specifically disrupt paxillin binding to each docking site on the FAT domain of FAK individually or in combination.” The paxillin binding sites are at the interface of ?-helices 1/4 and the interface of ?-helices 2/3 within the FAT domain (4 helices total). Paxillin is a scaffolding protein containing multiple domains that mediate protein-protein interactions, including five N-terminal LD motifs, four C-terminal LIM domains, and SH2 and SH3 domain binding sites. The second (LD2) and fourth LD motifs (LD4) of paxillin have been identified as FAK-binding sites and each of these sites binds to FAK with similar affinity.

Okay - on to the data.

With GFP-tagging, immunofluorescence, and TIRF microscopy, and Scheswohl et al. take a look at the differential expression of FAK mutants, demonstrating a somewhat cooperative mechanism for the two Paxillin-binding regions in facilitating focal adhesion targeting of FAK. Also, they analyzed tyrosine phosphorylation levels at three of the major activation regions, including the autophosphorylation site, kinase activating region, and the C-terminal Grb2-binding region and found similar results.

This is a relatively simple finding that the authors don’t take much further - the Paxillin-binding sites in the FAT domain of FAK are not redundant. Further, from the TIRF images, it looks as though FAK is lost from different classes of focal adhesions, depending upon whether helices 1/4 (I937A) or 2/3 (EKR) are mutated (see Figure 2C above).

Why these interactions differ is not clear. That cells would acquire separate mechanisms to spatially distinguish what type of focal adhesion they localize to would, likely benefit seems the cells, and thus the opportunist cell might acquire a mechanism to do so. But that’s mechanics - both seem equally capable and equally necessary for all biochemical pathways examined. Why no division of labor there?

Just Science