title: The RNA binding protein MEX-3 retains asymmetric activity in the early Caenorhabditis elegans embryo in the absence of asymmetric protein localization.
abstract: The RNA binding protein MEX-3 is required to restrict translation of pal-1, the Caenorhabditis elegans caudal homolog, to the posterior of the early embryo. MEX-3 is present uniformly throughout the newly fertilized embryo, but becomes depleted in the posterior by the 4-cell stage. This MEX-3 patterning requires the CCCH zinc-finger protein MEX-5, the RNA Recognition Motif protein SPN-4, and the kinase PAR-4. Genetic and biochemical evidence suggests that MEX-5 binds to MEX-3 in the anterior of the embryo, protecting MEX-3 from degradation and allowing it to bind the pal-1 3'UTR and repress translation. MEX-3 that is not bound to MEX-5 becomes inactivated by par-4, then targeted for spn-4 dependent degradation. After the 4-cell stage, residual MEX-3 is degraded in somatic cells, and only persists in the germline precursors. To better understand regulation of mex-3, GFP was fused to MEX-3 or regions of MEX-3 and expressed in developing oocytes. GFP::MEX-3 expressed in this manner can replace endogenous MEX-3, but surprisingly is not asymmetrically localized at the 4-cell stage. These results indicate that GFP::MEX-3 retains asymmetric activity even in the absence of asymmetric protein localization. Neither the mex-3 3'UTR nor protein degradation at the 4-cell stage is strictly required. A region of MEX-3 containing a glutamine-rich region and potential ubiquitination and phosphorylation sites is sufficient for soma-germline asymmetry. Results from mex-5/6 and spn-4(RNAi) suggest two pathways for MEX-3 degradation, an early spn-4 dependent pathway and a later spn-4 independent pathway. These results indicate that mex-3 activity is regulated at multiple levels, leading to rapid and robust regulation in the quickly developing early embryo.
authors.affiliation: Molecular Biology Department, Colorado College, Colorado Springs, CO 80903, USA. Electronic address: Nancy.Huang@ColoradoCollege.edu.
authors.forename: Nancy N
authors.initials: NN
authors.lastname: Huang
authors.affiliation: Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA. Electronic address: cphunter@g.harvard.edu.
authors.forename: Craig P
authors.initials: CP
authors.lastname: Hunter
mesh_terms: D020413:3' Untranslated Regions; D000595:Amino Acid Sequence; D000818:Animals; D017173:Caenorhabditis elegans / Q000196:embryology* / Q000378:metabolism; D029742:Caenorhabditis elegans Proteins / Q000235:genetics* / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism; D004625:Embryo, Nonmammalian; D018507:Gene Expression Regulation, Developmental; D005854:Germ Cells / Q000378:metabolism; D010766:Phosphorylation; D059748:Proteolysis; D016601:RNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D015398:Signal Transduction; D054875:Ubiquitination
chemical_list: D020413:3' Untranslated Regions; D029742:Caenorhabditis elegans Proteins; D018797:Cell Cycle Proteins; C102175:MEX-3 protein, C elegans; C410587:MEX-5 protein, C elegans; C410589:MEX-6 protein, C elegans; D016601:RNA-Binding Proteins; C442231:SPN-4 protein, C elegans
keywords: 3′UTR; MEX-5; PAL-1; Pattern formation; SPN-4; Translational control