Sadeghian, F.; Cram, E. J.

Proper regulation of contraction and relaxation in biological tubes is essential for organismal function. In C. elegans, the spermatheca, composed of smooth muscle-like cells, undergoes repeated stretching and contraction as oocytes pass through. Here we describe PES-8, a previously uncharacterized protein, as a regulator of spermatheca contractility. PES-8 contains a predicted extracellular zona pellucida-like domain and an unstructured cytoplasmic tail, suggesting dual roles in extracellular and cytoplasmic signaling. PES-8 localizes to the plasma membrane of the spermatheca, the spermathecal-uterine valve, and uterus. Functional analysis shows that PES-8 is essential for spermathecal function; its loss disrupts actomyosin fiber alignment, FLN-1/filamin localization, apical junction organization, and calcium signaling, preventing oocyte transit. These findings identify PES-8 as a key regulator of cytoskeletal organization and calcium-mediated contractility in the C. elegans spermatheca.