Two Caenorhabditis elegans calponin-related proteins have overlapping functions to maintain cytoskeletal integrity and are essential for reproduction

Multicellular organisms have multiple genes encoding calponins and calponin-related proteins, and some of these are known to regulate actin cytoskeletal dynamics and contractility. However, functional similarities and differences among these proteins are largely unknown. In the nematode Caenorhabditis elegans, UNC-87 is a calponin-related protein with seven calponin-like (CLIK) motifs and is required for maintenance of contractile apparatuses in muscle cells. Here, we report that CLIK-1, another calponin-related protein that also contains seven CLIK motifs, has an overlapping function with UNC-87 to maintain actin cytoskeletal integrity in vivo and has both common and different actin-regulatory activities in vitro. CLIK-1 is predominantly expressed in the body wall muscle and somatic gonad, where UNC-87 is also expressed. unc-87 mutation causes cytoskeletal defects in the body wall muscle and somatic gonad, whereas clik-1 depletion alone causes no detectable phenotypes. However, simultaneous depletion of clik-1 and unc-87 caused sterility due to ovulation failure by severely affecting the contractile actin networks in the myoepithelial sheath of the somatic gonad. In vitro, UNC-87 bundles actin filaments. However, CLIK-1 binds to actin filaments without bundling them and is antagonistic to UNC-87 in filament bundling. UNC-87 and CLIK-1 share common functions to inhibit cofilin binding and allow tropomyosin binding to actin filaments, suggesting that both proteins stabilize actin filaments. Thus, partially redundant functions of UNC-87 and CLIK-1 in ovulation is likely mediated by their common actin-regulatory activities, but their distinct activities in actin bundling suggest that they also have different biological functions.