Preserving the immortality of the germline: Germ cells are distinct from somatic cells in being immortal. MES-2, MES-3, MES-4, and MES-6 are required for survival and proliferation of the germline and thus help protect germline immortality. MES-2 and MES-6 are homologs of Polycomb-group transcriptional repressors in Drosophila. MES-2 and MES-6 interact directly and function in a complex with MES-3. MES-4 functions separately, and shows the striking behavior of binding to the autosomes but not the X chromosomes. MES-2, MES-3, and MES-6 are required to exclude MES-4 and also markers of actively-expressed chromatin from the X chromosomes. Our findings strengthen the emerging view that in the C. elegans germline, the X chromosomes differ in chromatin state from the autosomes and are generally silenced. Based on our studies of the MES proteins and the sensitivity of the Mes mutant phenotype to X-chromosome dosage, we think that the primary function of the MES system is to repress gene expression from the X chromosomes. The MES-2/MES-3/MES-6 complex may participate directly in repression. MES-4's role may be to exclude repressors from the autosomes, thus enabling efficient repression of the Xs. We are testing this model.

Germline proliferation and differentiation: "Germ granules" are distinctive organelles found in the germ cells of many species, including C. elegans. They have been invoked as "instructors" of germline development ever since their dramatic segregation to the germline was first observed. We have demonstrated that the germ granules in C. elegans are indeed required for fertility. We have now identified quite a few germ-granule proteins (PGL-1, PGL-2, PGL-3, and in collaboration with Karen Bennett's lab GLH-1, GLH-2, GLH-3, and GLH-4), all of which are predicted to bind RNA and several of which have been shown to be required for germline proliferation and/or formation of eggs and sperm. Our newest P-granule protein is eIF4E, the component of the translation initiation complex that binds to mRNA caps. Our working model is that germ granules control the translation and perhaps the localization and stability of mRNAs in the germline. We are working to define the pathway of assembly of P granules and the roles of individual P-granule components.

Roles of microtubule motors in early embryo development: In a collaboration with Bill Saxton's lab, we are investigating the roles in early embryos of the ~20 motor proteins (kinesins and dyneins) that carry cargoes along microtubules. By eliminating the functions of individual motors, we have identified several that are involved in spindle formation, chromosome segregation, and cytokinesis. We are currently comparing the behavior of GFP-tagged microtubules and GFP-tagged chromosomes in living wild-type and motor-depleted embryos - the movies are very informative and cool to watch!