Caenorhabditis elegans gelsolin-like protein-1 (GSNL-1) is a new member of the gelsolin family of actin regulatory proteins [Klaavuniemi, T., Yamashiro, S., and Ono, S. (2008) J. Biol. Chem. 283, 26071-26080]. It is an unconventional gelsolin-related protein with four gelsolin-like (G) domains (G1-G4), unlike typical gelsolin-related proteins with three or six G domains. GSNL-1 severs actin filaments and caps the barbed end in a calcium-dependent manner similar to that of gelsolin. In contrast, GSNL-1 has properties different from those of gelsolin in that it remains bound to F-actin and does not nucleate actin polymerization. To understand the mechanism by which GSNL-1 regulates actin dynamics, we investigated the domain-function relationship of GSNL-1 by analyzing activities of truncated forms of GSNL-1. G1 and the linker between G1 and G2 were sufficient for actin filament severing, whereas G1 and G2 were required for barbed end capping. The actin severing activity of GSNL-1 was inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2), and a PIP2-sensitive domain was mapped to G1 and G2. At least two actin-binding sites were detected: a calcium-dependent G-actin-binding site in G1 and a calcium-independent G- and F-actin-binding site in G3 and G4. These results reveal both conserved and different utilization of G domains between C. elegans GSNL-1 and mammalian gelsolin for actin regulatory functions.