Subcellular patterns of SPE-6 localization reveal unexpected complexities in Caenorhabditis elegans sperm activation and sperm function

To acquire and maintain directed cell motility, Caenorhabditis elegans sperm must undergo extensive, regulated cellular remodeling, in the absence of new transcription or translation. To regulate sperm function, nematode sperm employ large numbers of protein kinases and phosphatases, including SPE-6, a member of C. elegans' highly expanded casein kinase 1 superfamily. SPE-6 functions during multiple steps of spermatogenesis, including functioning as a "brake" to prevent premature sperm activation in the absence of normal extracellular signals. Here, we describe the subcellular localization patterns of SPE-6 during wild-type C. elegans sperm development and in various sperm activation mutants. While other members of the sperm activation pathway associate with the plasma membrane or localize to the sperm's membranous organelles, SPE-6 surrounds the chromatin mass of unactivated sperm. During sperm activation by either of two semiautonomous signaling pathways, SPE-6 redistributes to the front, central region of the sperm's pseudopod. When disrupted by reduction-of-function alleles, SPE-6 protein is either diminished in a temperature-sensitive manner (hc187) or is mislocalized in a stage-specific manner (hc163). During the multistep process of sperm activation, SPE-6 is released from its perinuclear location after the spike stage in a process that does not require the fusion of membranous organelles with the plasma membrane. After activation, spermatozoa exhibit variable proportions of perinuclear and pseudopod-localized SPE-6, depending on their location within the female reproductive tract. These findings provide new insights regarding SPE-6's role in sperm activation and suggest that extracellular signals during sperm migration may further modulate SPE-6 localization and function.