Microscopic Evidence That Actin-interacting Protein 1 Actively Disassembles Actin-depolymerizing Factor/Cofilin-bound Actin Filaments

Abstract

Actin-depolymerizing factor (ADF)/cofilin and gelsolin are the two major factors to enhance actin filament disassembly. Actin-interacting protein 1 (AIP1) enhances fragmentation of ADF/cofilin-bound filaments and caps the barbed ends. However, the mechanism by which AIP1 disassembles ADF/cofilin-bound filaments is not clearly understood. Here, we directly observed the effects of these proteins on filamentous actin by fluorescence microscopy and gained novel insight into the function of ADF/cofilin and AIP1. ADF/cofilin severed filaments and AIP1 strongly enhanced disassembly at nanomolar concentrations. However, gelsolin, gelsolinactin complex, or cytochalasin D did not enhance disassembly by ADF/cofilin, suggesting that the strong activity of AIP1 cannot be explained by simple barbed end capping. Barbed end capping by ADF/cofilin and AIP1 was weak and allowed filament elongation, whereas gelsolin or gelsolin-actin complex strongly capped and inhibited elongation. These results suggest that AIP has an active role in filament severing or depolymerization and that ADF/cofilin and AIP1 are distinct from gelsolin in modulating filament elongation.

A Genetic Dissection of Aip1p's Interactions Leads to a Model for Aip1p-Cofilin Cooperative Activities

“…Although these disparities may be specific to the organism from which the Aip1p is obtained, it is possible that the weaker activities observed for UNC-78 and XAip1 have resulted from the use of rabbit skeletal muscle actin in the disassembly assays, whereas yeast Aip1p was tested using yeast actin. The ability of Aip1p to enhance severing of cofilin-decorated actin filaments has been convincingly documented (Okada et al, 1999;Ono et al, 2004). In addition, some findings indicate that Aip1p is able to cap the barbed ends of actin filaments, preventing filament elongation and thus enhancing disassembly (Okada et al, 2002;Balcer et al, 2003).…”

“…The biochemical activities of Aip1p from multiple sources (S. cerevisiae, X. laevis, and C. elegans) have been extensively tested in vitro (Okada et al, 1999(Okada et al, , 2002Rodal et al, 1999;Balcer et al, 2003;Mohri and Ono, 2003;Mohri et al, 2004;Ono et al, 2004). In actin pelleting assays, S. cerevisiae Aip1p very strongly induces the disassembly of cofilin-bound actin filaments, whereas C. elegans Aip1p (UNC-78) does so moderately, and X. laevis Aip1p (XAip1) does so very weakly (Okada et al, 1999;Rodal et al, 1999;Mohri and Ono, 2003).…”

“…Biochemical assays show that cofilin promotes depolymerization of actin filaments by accelerating pointed-end filament disassembly (Carlier et al, 1997;Lappalainen and Drubin, 1997) and by a severing activity that is very weak at physiological pH (Maciver et al, 1991;Ichetovkin et al, 2000). Aip1p dramatically enhances the depolymerization activity of cofilin-decorated actin filaments and is suspected to do so by assisting cofilin-induced severing and/or by capping the barbed ends of actin filaments (Okada et al, 1999;Rodal et al, 1999;Okada et al, 2002;Balcer et al, 2003;Mohri et al, 2004;Ono et al, 2004).…”

The flare Gene, Which Encodes the AIP1 Protein of Drosophila, Functions to Regulate F-Actin Disassembly in Pupal Epidermal Cells

“…AIP1 has been found to bind directly to both actin and cofilin, but it is not thought that it has any actin depolymerizing activity on its own (Ono 2003;Ono et al 2004;Rodal et al 2005;Clark et al 2006;Okada et al 2006). In yeast cofilin is an essential gene, while AIP1 is not.…”

“…In yeast, animals, and plants the actin interacting protein 1 (AIP1) facilitates the ability of cofilin to promote actin depolymerization (Okada et al 1999(Okada et al , 2006Balcer et al 2003;Mohri and Ono 2003;Ono 2003;Ketelaar et al 2004;Ono et al 2004;Paavilainen et al 2004;Rodal et al 2005;Clark et al 2006). AIP1 has been found to bind directly to both actin and cofilin, but it is not thought that it has any actin depolymerizing activity on its own (Ono 2003;Ono et al 2004;Rodal et al 2005;Clark et al 2006;Okada et al 2006).…”