Sperm-specific fertility factors SMZ-1/2 promote FB-MO biogenesis and MSP filament assembly in C. elegans

In nematode sperm, motility is powered by polymerization of major sperm protein (MSP) into filaments assembled from a specialized organelle (FB-MO), but how filament formation begins remains unclear. We identify two sperm-specific PDZ proteins, SMZ-1 and SMZ-2 (SMZ-1/2), that initiate MSP loading into pre-formed membranous organelles in Caenorhabditis elegans. SMZ-1/2 were expressed specifically in spermatogenic germ cells, colocalized with developing FB-MOs from diplotene through spermatid formation, and were redundant for fertility. Loss of SMZ-1/2 abolished MSP filaments, left MOs lacking MSP filaments that are morphologically abnormal, and caused primary spermatocyte arrest. Electron microscopy showed fewer, smaller FB-MOs lacking crystalline FBs, consistent with a defect in assembly initiation rather than late polymerization. SMZ-1/2 cytoplasmic structures were absent in spe-6 casein kinase mutants, placing SMZ-1/2 within a SPE-6-dependent initiation module. These findings identify SMZ-1/2 as PDZ scaffold proteins that initiate MSP assembly and couple kinase signaling to FB-MO biogenesis during nematode spermatogenesis.