THE C-ELEGANS MEX-1 PROTEIN IS PRESENT IN GERMLINE BLASTOMERES AND ISA P-GRANULE COMPONENT

In the nematode Caenorhabditis elegans, germ cells arise from early em bryonic cells called germline blastomeres. Cytoplasmic structures call ed P granules are present in the fertilized egg and are segregated int o each of the germline blastomeres during the first few cleavages of t he embryo. Mutations in the maternally expressed gene mex-1 disrupt th e segregation of P granules, prevent the formation of germ cells, and cause inappropriate patterns of somatic cell differentiation. We have cloned the mex-1 gene and determined the distribution pattern of the m ex-1 gene products. The MEX-1 protein contains two copies of an unusua l 'finger' domain also found in the PIE-1 protein of C. elegans. PIE-1 has been shown to be expressed in germline blastomeres, and is a comp onent of P granules. We show here that MEX-1 also is present in germli ne blastomeres and is a P granule component, although MEX-1 is a cytop lasmic protein while PIE-1 is present in both the nucleus and cytoplas m. We further show that MEX-1 is required to restrict PIE-1 expression and activity to the germline blastomeres during the early embryonic c leavages.