Early C. elegans embryos exhibit protein asymmetries that allow rapid diversification of cells. Establishing these asymmetries requires the novel protein MEX-5. We show that mutations in the efl-1 and dpl-1 genes cause defects in protein localization resembling defects caused by mutations in mex-5. efl-1 and dpl-1 encode homologs of vertebrate E2F and DP proteins that regulate transcription as a heterodimer. efl-1 and dpl-1 mutants have elevated levels of activated Map kinase in oocytes. Their mutant phenotype and that of mex-5 mutants can be suppressed by reducing Ras/Map kinase signaling. We propose this signaling pathway has a role in embryonic asymmetry and that EFL-1/DPL-1 control the level of Map kinase activation.

Atrophin-1-interacting protein 4 (AIP4) is the human homolog of the mouse Itch protein (hItch), an E3 ligase for Notch and JunB. Human enhancer of filamentation 1 (HEF1) has been implicated in signaling pathways such as those mediated by integrin, T cell receptor, and B cell receptor and functions as a multidomain docking protein. Recent studies suggest that HEF1 is also involved in the transforming growth factor-␤ (TGF-␤) signaling pathways, by interacting with Smad3, a key signal transducer downstream of the TGF-␤ type I receptor. The interaction of Smad3 with HEF1 induces HEF1 proteasomal degradation, which was further enhanced by TGF-␤ stimulation. The detailed molecular mechanisms of HEF1 degradation regulated by Smad3 were poorly understood. Here we report our studies that demonstrate the function of AIP4 as an ubiquitin E3 ligase for HEF1. AIP4 forms a complex with both Smad3 and HEF1 through its WW domains in a TGF-␤-independent manner and regulates HEF1 ubiquitination and degradation, which can be enhanced by TGF-␤ stimulation. These findings reveal a new mechanism for Smad3-regulated proteasomal degradation events and also broaden the network of cross-talk between the TGF-␤ signaling pathway and those involving HEF1 and AIP4.The transforming growth factor-␤ (TGF-␤) 1 signaling is involved in a broad range of cellular functions, including proliferation, adhesion, apoptosis, differentiation, and specification of developmental fate (1, 2). The extracellular signals were transduced to the nucleus by the sequential association of type II and type I receptors and the Smad protein cascades (3, 4). The binding of ligands to the receptors leads to the phosphorylation of Smad2 and Smad3 at their SSXS motif within the COOH termini. The phosphorylated Smad2 or Smad3 forms complexes with Smad4 and translocates into the nucleus, where they function as DNA-binding transcription factors. Recently, Smad3 was discovered to have the novel ability of regulating the proteasomal degradation of the nuclear proto-oncoproteins SnoN and Ski (5, 6) as well as the human enhancer of filmentation 1 (HEF1) (7).HEF1 is a member of a multiple domain docking protein Cas family including p130 cas and Efs that have been implicated as signaling mediators of diverse processes including cellular attachment, motility, growth factor responses, apoptosis, and oncogenic transformation (8). HEF1 was first isolated in a screen for human proteins with the ability to alter Saccharomyces cerevisiae morphology from round to filamentous hyperpolarized cells (9). Based upon its homology to p130 cas , another group independently isolated HEF1, named Cas-L (10). Members of this family share similar domains, with an aminoterminal Src homology 3 domain that binds polyproline-containing protein, a large central domain encompassing multiple tyrosine motifs that are recognized by the Src homology 2 domain protein upon phosphorylation, a serine-rich domain, and a carboxyl-terminal domain containing a helix-loop-helix motif (9, 11).Earlier studies have showed t...

In the nematode Caenorhabditis elegans, germ cells arise from early embryonic cells called germline blastomeres. Cytoplasmic structures called P granules are present in the fertilized egg and are segregated into each of the germline blastomeres during the first few cleavages of the embryo. Mutations in the maternally expressed gene mex-1 disrupt the 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 mex-1 gene products. The MEX-1 protein contains two copies of an unusual ‘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 component of P granules. We show here that MEX-1 also is present in germline blastomeres and is a P granule component, although MEX-1 is a cytoplasmic protein while PIE-1 is present in both the nucleus and cytoplasm. We further show that MEX-1 is required to restrict PIE-1 expression and activity to the germline blastomeres during the early embryonic cleavages.