title: Multiple RNA-binding proteins function combinatorially to control the soma-restricted expression pattern of the E3 ligase subunit ZIF-1.
abstract: In C. elegans embryos, transcriptional repression in germline blastomeres requires PIE-1 protein. Germline blastomere-specific localization of PIE-1 depends, in part, upon regulated degradation of PIE-1 in somatic cells. We and others have shown that the temporal and spatial regulation of PIE-1 degradation is controlled by translation of the substrate-binding subunit, ZIF-1, of an E3 ligase. We now show that ZIF-1 expression in embryos is regulated by five maternally-supplied RNA-binding proteins. POS-1, MEX-3, and SPN-4 function as repressors of ZIF-1 expression, whereas MEX-5 and MEX-6 antagonize this repression. All five proteins bind directly to the zif-1 3' UTR in vitro. We show that, in vivo, POS-1 and MEX-5/6 have antagonistic roles in ZIF-1 expression. In vitro, they bind to a common region of the zif-1 3' UTR, with MEX-5 binding impeding that by POS-1. The region of the zif-1 3' UTR bound by MEX-5/6 also partially overlaps with that bound by MEX-3, consistent with their antagonistic functions on ZIF-1 expression in vivo. Whereas both MEX-3 and SPN-4 repress ZIF-1 expression, neither protein alone appears to be sufficient, suggesting that they function together in ZIF-1 repression. We propose that MEX-3 and SPN-4 repress ZIF-1 expression exclusively in 1- and 2-cell embryos, the only period during embryogenesis when these two proteins co-localize. As the embryo divides, ZIF-1 continues to be repressed in germline blastomeres by POS-1, a germline blastomere-specific protein. MEX-5/6 antagonize repression by POS-1 and MEX-3, enabling ZIF-1 expression in somatic blastomeres. We propose that ZIF-1 expression results from a net summation of complex positive and negative translational regulation by 3' UTR-binding proteins, with expression in a specific blastomere dependent upon the precise combination of these proteins in that cell.
authors.affiliation: Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
authors.forename: Marieke
authors.initials: M
authors.lastname: Oldenbroek
authors.forename: Scott M
authors.initials: SM
authors.lastname: Robertson
authors.forename: Tugba
authors.initials: T
authors.lastname: Guven-Ozkan
authors.forename: Steven
authors.initials: S
authors.lastname: Gore
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Nishi
authors.forename: Rueyling
authors.initials: R
authors.lastname: Lin
mesh_terms: D020413:3' Untranslated Regions; D000818:Animals; D001757:Blastomeres / Q000378:metabolism; D017173:Caenorhabditis elegans / Q000196:embryology / Q000378:metabolism; D029742:Caenorhabditis elegans Proteins / Q000096:biosynthesis*; D002352:Carrier Proteins / Q000096:biosynthesis*; D018507:Gene Expression Regulation, Developmental; D011485:Protein Binding; D016601:RNA-Binding Proteins / Q000378:metabolism*
chemical_list: D020413:3' Untranslated Regions; D029742:Caenorhabditis elegans Proteins; D002352:Carrier Proteins; D016601:RNA-Binding Proteins; C476529:ZIF-1 protein, C elegans