Capping Protein Is a Membrane-Associated Actin-Binding Protein in Arabidopsis.

Capping Protein is a Membrane-Associated Actin-Binding Protein in Arabidopsis J.C. Jimenez-Lopez, X. Wang, S. Huang, C.J. Staiger Department of Biological Sciences, Purdue University, West Lafayette, IN USA The actin cytoskeleton is a dynamic structure, and a major regulator of relevant processes such as cell morphogenesis, sexual reproduction and cellular responses to extracellular stimuli. Understanding the structural and functional organization of the plant cytoskeleton is of crucial importance for agriculture food production and safety. Cytoskeletal function is coordinated through a plethora of actin-binding proteins (ABPs). Many of these proteins are regulated directly by binding to phospholipids. Heterodimeric capping protein (CP) is a major actin cytoskeleton regulator; it binds to filament plus-ends with high affinity and regulates filament assembly and disassembly reactions. Filament ends can be uncapped by direct interaction of CP with phosphatidic acid, a major component of plant membranes. Whether CP associates with membranes in plant cells remains an open question. Complementary biochemical approaches were used to estimate CP cellular abundance and to elucidate possible CP-membrane association. CP was demonstrated to be moderately abundant in the cell, but likely sufficient to cap all available filament ends. Differential centrifugation and sucrose density gradients provide initial evidence that CP associates with membrane-bound organelles. This association may have profound implications for many regulated membrane functions. For example, it may facilitate mediating crosstalk between the actin cytoskeleton and a wide spectrum of essential cellular functions and processes. In particular, it may promote regulated actin assembly on cellular compartments, thereby enhancing processes like intracellular vesicle trafficking, endocytosis and post-endocytic traffic through the endosomal membrane system. The new knowledge acquired from this work will have a positive impact to alter beneficial properties of agronomically important plant species. This study was funded by the Physical Biosciences program of the Office of Basic Energy Science, US Department of Energy under contract number DE-FG02-04ER15526.