Actin Crosslinking Proteins Localize Based on Regional Affinities for F-actin and Contribute to Normal Cellular Motility and Morphology

A functional actin cytoskeleton is essential to life for nearly all organisms. Proteins that crosslink individual actin filaments into networks (ACLP’s) are required for normal cytoskeletal function. The aim of this study was to define the mechanisms by which ACLP’s are dynamically localized in cells and how these proteins contribute to normal cellular motility and morphology. We established that 2 ACLP’s, αA and FLN, associate discriminately with the F-actin cytoskeleton. Photo-convertible fluorescent protein fusions to αA and FLN demonstrated that both proteins exhibit relatively dynamic binding, yet significant differences in F-actin binding affinity can be measured in some cellular locations. We determined that the distinct localizations of αA and FLN are not due to direct competition between the two proteins, or to steric hindrance caused by the F-actin cytoskeleton itself or differences in the tertiary structure of either protein. Hence the localization patterns of αA and FLN seem to be the result of locally specific affinities for F-actin in different parts of the cell. Lastly, we have shown that FLN plays a role in allowing cells to move more quickly, most likely through facilitation of cell shape change. αA, Fimbrin (Fim) and actin binding protein-34 (ABP34) function to allow faster cell movement and proper polarization, especially in restrictive environments. This work has demonstrated that ACLP’s seem to play specific rather than general roles in cytoskeletal function and that ACLP’s are localized to their appropriate places in the cell by a local and specific affinity for F-actin in different cellular locations.