The Two Caenorhabditis elegans Actin-Depolymerizing Factor/Cofilin Proteins Differently Enhance Actin Filament Severing and Depolymerization

Abstract

Actin-depolymerizing factor (ADF)/cofilin enhances the turnover of actin filaments by two separable activities: filament severing and pointed-end depolymerization. Multicellular organisms express multiple ADF/cofilin isoforms in a tissue-specific manner, and the vertebrate proteins are grouped into ADFs and cofilins on the basis of their biochemical activity. A recent comparative study has shown that ADF has greater severing and depolymerizing activities than cofilin [Chen, H., Bernstein, B. W., Sneider, J. M., Boyle, J. A., Minamide, L. S., and Bamburg, J. R. (2004) Biochemistry 43, 7127-7142]. Here, we show that the two Caenorhabditis elegans ADF/cofilin isoforms exhibit different activities for severing and depolymerizing actin filaments. The ADF-like non-muscle isoform UNC-60A had greater activities to cause net depolymerization and inhibit polymerization than the cofilin-like muscle isoform UNC-60B. Surprisingly, UNC-60B exhibited much stronger severing activity than UNC-60A, which was the opposite of what was observed for vertebrate counterparts. Moreover, UNC-60B induced much faster pointed-end depolymerization of rabbit muscle actin than UNC-60A, while UNC-60A caused slightly faster depolymerization of C. elegans actin than UNC-60B. These results suggest that cofilin-like UNC-60B is kinetically more efficient in enhancing actin turnover than ADF-like UNC-60A, while ADF-like UNC-60A is suitable for maintaining higher concentrations of monomeric actin. These functional differences might be specifically adapted for different actin dynamics in muscle and non-muscle cells.

CAS-1, a C. elegans cyclase-associated protein, is required for sarcomeric actin assembly in striated muscle

“…4B, black curve) as demonstrated previously (Yamashiro et al, 2005). However, this inhibitory effect of UNC-60B was relieved by MBP-CAS-1 (Fig.…”

“…S3A,C, lanes 4, 6 and 8). Based on our previous studies (Yamashiro et al, 2005), UNC-60B is expected to bind to actin under these conditions. Therefore, lack of UNC-60B in the precipitates suggests that UNC-60B was dissociated from actin in the presence of MBP-CAS-1 or MBP-CAS-1C.…”

Actin-interacting Protein 1 Promotes Disassembly of Actin-depolymerizing Factor/Cofilin-bound Actin Filaments in a pH-dependent Manner

“…In these experiments, an excessive concentration of UNC-60B (20 M) was added to F-actin (10 M). Under these conditions, although UNC-60B weakly severs actin filaments, it does not sequester actin monomers, and UNC-60B-severed filaments are still long enough to be sedimented by ultracentrifugation (49).…”

“…UNC-60A has very weak actin filament-severing and strong monomer-sequestering activities, whereas UNC-60B has strong actin filament-severing and nearly no monomer-sequestering activities (48,49). Both of them show only weak pH sensitivity in actin filament disassembly when filament severing and depolymerization are examined in solution (49). In this study, we found two C. elegans AIP1 isoforms, UNC-78 and AIPL-1 (50 -52), have pH sensitivity in disassembly of UNC-60B-bound actin filaments.…”

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…The forms differ in their filament severing and depolymerizing abilities and tissue locations. UNC-60A is required for proper early development and UNC-60B for muscle sarcomere structure (Ono and Benian, 1998;Ono et al, 1999Ono et al, , 2003Ono, 2003;Yamashiro et al, 2005). The C-terminal portion of UNC-60B is critical for its interactions with filamentous actin .…”