TwoCaenorhabditis eleganscalponin-related proteins have overlapping functions that maintain cytoskeletal integrity and are essential for reproduction

Multicellular organisms have multiple genes encoding calponins and calponin-related proteins, some of which are known to regulate actin cytoskeletal dynamics and contractility. However, the functional similarities and differences among these proteins are largely unknown. In the nematodeCaenorhabditis 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, functionally overlaps with UNC-87 in maintaining actin cytoskeletal integrityin vivoand has both common and different actin-regulatory activitiesin vitro. We found that CLIK-1 is predominantly expressed in the body wall muscle and somatic gonad in which UNC-87 is also expressed.unc-87mutation caused cytoskeletal defects in the body wall muscle and somatic gonad, whereasclik-1depletion alone caused no detectable phenotypes. However, simultaneousclik-1andunc-87depletion caused sterility because of ovulation failure by severely affecting the contractile actin networks in the myoepithelial sheath of the somatic gonad.In vitro, UNC-87 bundled actin filaments, whereas CLIK-1 bound to actin filaments without bundling them and antagonized UNC-87-mediated filament bundling. We noticed that UNC-87 and CLIK-1 share common functions that inhibit cofilin binding and allow tropomyosin binding to actin filaments, suggesting that both proteins stabilize actin filaments. In conclusion, partially redundant functions of UNC-87 and CLIK-1 in ovulation are likely mediated by their common actin-regulatory activities, but their distinct actin-bundling activities suggest that they also have different biological functions.