FAP52 – a new partner for filamin in actin cytoskeleton organization

Abstract

Focal-adhesion-associated phosphoprotein 52 (FAP52), as its name suggests, colocalizes with other adhesion proteins such as vinculin, talin and paxillin. It is homologous with two other proteins, PASCIN2 and syndapin II, all three proteins sharing an FER-CIP4 homology (FCH) domain followed by an α-helical region and a Src-homology 3 (SH3) domain. The specific functions of FAP52 and PASCIN2 are not known, but the involvement of syndapin II in actin organization and endocytosis hints at an exciting role for its two relatives. Sure enough, Nikki et al. now show that FAP52 directly binds to the actin-crosslinking protein filamin (ABP-280) [1xSee all References][1].Pulldown experiments with cell extracts allowed the identification of filamin-interacting proteins by nanoelectrospray tandem mass spectrometry. Coupling this with surface plasmon resonance analyses revealed the minimal binding site between filamin and FAP52, showing that amino acids 1–184 of FAP52 are essential for its binding to filamin. These amino acids correspond to the FCH domain and helical region of FAP52. Similarly, spanning repeats 15–16 (amino acids 1664–1858) of filamin are necessary to bind to FAP52. The results of immunofluorescence and immunoelectron microscopy of cultured cells shows that FAP52 and filamin colocalize at sites where actin stress fibers contact focal adhesions. Discussing this interaction, Nikki and colleagues suggest that the binding could affect both the flexibility of the filamin dimer and its ability to crosslink the actin network. This leads to the question of how these two proteins function mechanistically with respect to actin cytoskeleton dynamics.This paper represents a very elegant elucidation of the binding of FAP52 and filamin based on the use of divergent and complementary biochemical methods. By using this ‘reverse strategy’, the authors have revealed a role for FAP52 in linking focal adhesions to the actin cytoskeleton without prior knowledge – a genuinely ‘blind’ experiment.