Actin binding domain of filamin distinguishes posterior from anterior actin filaments in migrating Dictyostelium cells

Actin filaments in different parts of a cell interact with specific actin binding proteins (ABPs) and perform different functions in a spatially regulated manner. However, the mechanisms of those spatially-defined interactions have not been fully elucidated. If the structures of actin filaments differ in different parts of a cell, as suggested by previous in vitro structural studies, ABPs may distinguish these structural differences and interact with specific actin filaments in the cell. To test this hypothesis, we followed the translocation of the actin binding domain of filamin (ABD(FLN)) fused with photoswitchable fluorescent protein (mKikGR) in polarized Dictyostelium cells. When ABD(FLN)-mKikGR was photoswitched in the middle of a polarized cell, photoswitched ABD(FLN)-mKikGR rapidly translocated to the rear of the cell, even though actin filaments were abundant in the front. The speed of translocation (>3 μm/s) was much faster than that of the retrograde flow of cortical actin filaments. Rapid translocation of ABD(FLN)-mKikGR to the rear occurred normally in cells lacking GAPA, the only protein, other than actin, known to bind ABD(FLN). We suggest that ABD(FLN) recognizes a certain feature of actin filaments in the rear of the cell and selectively binds to them, contributing to the posterior localization of filamin.