Fertilization triggers egg activation and converts the egg into a developing embryo. The events of this egg-to-embryo transition typically include the resumption of meiosis, the reorganization of the cortical actin cytoskeleton, and the remodeling of the oocyte surface. The factors that regulate sperm-dependent egg-activation events are not well understood. Caenorhabditis elegans EGG-3, a member of the protein tyrosine phosphatase-like (PTPL) family, is essential for regulating cell-surface and cortex rearrangements during egg activation in response to sperm entry. Although fertilization occurred normally in egg-3 mutants, the polarized dispersal of F-actin is altered, a chitin eggshell is not formed, and no polar bodies are produced. EGG-3 is associated with the oocyte plasma membrane in a pattern that is similar to CHS-1 and MBK-2. CHS-1 is required for eggshell deposition, whereas MBK-2 is required for the degradation of maternal proteins during the egg-to-embryo transition. The localization of CHS-1 and EGG-3 are interdependent and both genes were required for the proper localization of MBK-2 in oocytes. Therefore, EGG-3 plays a central role in egg activation by influencing polarized F-actin dynamics and the localization or activity of molecules that are directly involved in executing the egg-to-embryo transition.

In the nematode worm C. elegans, individuals with mutations in the spe-9 gene produce spermatozoa with wild-type morphology and motility that cannot fertilize oocytes even after contact between gametes. Therefore, disruption of spe-9 function affects either gamete recognition, adhesion, signaling, and/or fusion. The spe-9 gene encodes a sperm transmembrane protein with an extracellular domain that contains ten epidermal growth factor-like repeats. A common feature of proteins that include epidermal growth factor-like motifs is their involvement in extracellular functions such as adhesive and ligand-receptor interactions. Additionally, the overall structure of the predicted SPE-9 protein is similar to that of ligands for the Notch/LIN-12/GLP-1 family of transmembrane receptors. These results suggest that SPE-9 functions in the specialized cell-cell interactions required for fertilization.

DYRKs are kinases that self-activate in vitro by auto-phosphorylation of a YTY motif in the kinase domain, but their regulation in vivo is not well understood. In C. elegans zygotes, MBK-2/DYRK phosphorylates oocyte proteins at the end of the meiotic divisions to promote the oocyte-to-embryo transition. Here we demonstrate that MBK-2 is under both positive and negative regulation during the transition. MBK-2 is activated during oocyte maturation by CDK-1-dependent phosphorylation of Serine 68, a residue outside of the kinase domain required for full activity in vivo. The pseudo-tyrosine phosphatases EGG-4 and EGG-5 sequester activated MBK-2 until the meiotic divisions by binding to the YTY motif and inhibiting MBK-2’s kinase activity directly, using a novel mixed-inhibition mechanism that does not involve tyrosine dephosphorylation. Our findings link cell cycle progression to MBK-2/DYRK activation and the oocyte-to-embryo transition.

The body surface of the adult fly is covered with a complex array of multicellular sensory organs arranged in a largely invariant spatial pattern. In most cases, the cells comprising each of these organs are the progeny of a single sensory organ precursor (SOP) cell (Hartenstein and Posakony 1989). SOPs are determined during the late larval and early pupal stages within undifferentiated epithelial sheets, the imaginal discs and histoblast nests, that ultimately give rise to the cuticular structures of the adult. The process of establishing the pattern of cells committed to the SOP fate, and thus the pattern of sensory organs, consists of at least two major steps. First, the competence to acquire the SOP cell fate is conferred on small groups of cells by the spatially restricted expression and activity of certain proneural genes, which encode transcriptional activators of the basic-helixloop-helix (bHLH) class. Then, within each of these proneural clusters, all but one of the competent cells are inhibited from expressing the SOP fate by local cell-cell interactions that depend on the activity of a second set of Present addresses: 1Department of Molecular and Cell Biology,