Zweifel, M. E. | bioRxiv Science

bioRxiv2025

A sperm-oocyte protein partnership required for egg activation in Caenorhabditis elegans

ABSTRACTFertilization triggers the completion of female meiosis and launches the oocyte-to-embryo transition. C. elegans spe-11 is one of the few known paternal-effect embryonic lethal genes. We report that the sperm protein, SPE-11, forms a complex with an oocyte protein, OOPS-1 (Oocyte Partner of SPE-11), and that the protein complex is required for the completion of meiosis, the block to polyspermy, and eggshell formation. Consistent with the molecular interaction of their encoded proteins, oops-1 and spe-11 exhibit identical null phenotypes, displaying defects in meiotic progression and cytokinesis. We show that the complex binds F-actin in the absence of other proteins and inhibits the nucleation of actin filaments in vitro. Thus, the OOPS-1-SPE-11 complex may function to promote F-actin-mediated meiotic cytokinesis. Both OOPS-1 and SPE-11 are intrinsically disordered proteins that are highly phosphorylated. Biochemical and genetic experiments define interactions with the protein phosphatase 1 homologs GSP-3/4, which appear to promote OOPS-1-SPE-11 function. Genetic results support a model in which the OOPS-1-SPE-11 complex interacts with the cortical EGG complex to promote meiotic cytokinesis and to activate synthesis of the eggshell.

developmental biology↗

bioRxiv2021

Formin's nucleation activity influences actin filament length

Formins stimulate actin polymerization by promoting both filament nucleation and elongation. Because nucleation and elongation draw upon a common pool of actin monomers, the rate at which each reaction proceeds influences the other. This interdependent mechanism determines the number of filaments assembled over the course of a polymerization reaction, as well as their equilibrium lengths. In this study, we used kinetic modeling and in vitro polymerization reactions to dissect the contributions of filament nucleation and elongation to the process of formin-mediated actin assembly. We found that the rates of nucleation and elongation evolve over the course of a polymerization reaction. The period over which each process occurs is a key determinant of the total number of filaments that are assembled, as well as their average lengths at equilibrium. Inclusion of formin in polymerization reactions speeds filament nucleation, thus increasing the number and shortening the lengths of filaments that are assembled over the course of the reaction. Although variations in elongation rates produce modest changes in the equilibrium lengths of formin-bound filaments, nucleation constitutes the primary mode of monomer consumption over the course of assembly. Sustained elongation of small numbers of formin-bound filaments therefore requires inhibition of nucleation via monomer sequestration and a low concentration of activated formin. Our results underscore the mechanistic advantage for keeping formins nucleation efficiency relatively low in cells, where unregulated actin assembly would produce deleterious effects on cytoskeletal dynamics. Under these conditions, differences in the elongation rates mediated by formin isoforms are most likely to impact the kinetics of actin assembly.

biophysics↗