A conserved RNA-protein complex component involved in physiological germline apoptosis regulation inC. elegans

Abstract

Two conserved features of oogenesis are the accumulation of translationally quiescent mRNA, and a high rate of stage-specific apoptosis. Little is understood about the function of this cell death. In C. elegans, apoptosis occurring through a specific 'physiological' pathway normally claims about half of all developing oocytes. The frequency of this germ cell death is dramatically increased by a lack of the RNA helicase CGH-1, orthologs of which are involved in translational control in oocytes and decapping-dependent mRNA degradation in yeast processing (P) bodies. Here, we describe a predicted RNAbinding protein, CAR-1, that associates with CGH-1 and Y-box proteins within a conserved germline RNA-protein (RNP) complex, and in cytoplasmic particles in the gonad and early embryo. The CGH-1/CAR-1 interaction is conserved in Drosophila oocytes. When car-1 expression is depleted by RNA interference (RNAi), physiological apoptosis is increased, brood size is modestly reduced, and early embryonic cytokinesis is abnormal. Surprisingly, if apoptosis is prevented car-1(RNAi) animals are characterized by a progressive oogenesis defect that leads rapidly to gonad failure. Elevated germ cell death similarly compensates for lack of the translational regulator CPB-3 (CPEB), orthologs of which function together with CGH-1 in diverse organisms. We conclude that CAR-1 is of critical importance for oogenesis, that the association between CAR-1 and CGH-1 has been conserved, and that the regulation of physiological germ cell apoptosis is specifically influenced by certain functions of the CGH-1/CAR-1 RNP complex. We propose that this cell death pathway facilitates the formation of functional oocytes, possibly by monitoring specific cytoplasmic events during oogenesis.

RAP55, a Cytoplasmic mRNP Component, Represses Translation in Xenopus Oocytes

“…Recently, diverse functions have been reported for C. elegans and Drosophila homologues of RAP55, CAR-1, and Trailer hitch (Tral), respectively (56). In C. elegans CAR-1 is required for embryonic cytokinesis, regulation of physiological germ line apoptosis, and organization of the endoplasmic reticulum (49,50,57). Depletion of CAR-1 leads to a defect in cytokinesis during early embryogenesis resulting from an anaphase spindle defect (57).…”

“…In Drosophila oocytes and nurse cells, Tral is associated with a subset of endoplasmic reticulum exit sites and is required for the proper secretion of proteins such as Gurken and Yolkless (58). Importantly, interactions between the RAP55 homologues, the Xp54 homologues, and the Y-box proteins are conserved in these organisms, indicating that RAP55 and its homologues constitute core components of mRNPs together with the Xp54 homologues and the Y-box proteins and execute various functions (49,50,56,58). In previous studies with somatic cells such as rabbit reticulocytes or mouse L cells, core components of cytoplasmic mRNPs consisted of the Y-box proteins and the poly(A)-binding protein (59,60).…”

“…In C. elegans during embryonic division, CAR-1 localizes to P granules, which are cytoplasmic particles enriched in the germ line precursors and contain poly(A) ϩ RNAs (49,50). In Xenopus, germ granules, which are also cytoplasmic granules in oocytes and eggs, are essential for germ line development (61).…”

Germ Cell Apoptosis and DNA Damage Responses

“…Inactivation of multiple proteins increases the level of germ cell apoptosis and it was argued that various RNA binding proteins, such as the RGG-box protein CAR-1, the DEAD box helicase CGH-1, and the CPEB ortholog CPB-3, prevent excessive apoptosis induction (Boag et al 2005 ) . However, inactivation of these and other genes by mutation or RNAi leads to additional germline defects, such as an extended pachytene zone, reduced progeny, full or partial sterility, or a generally misshapen germline (Audhya et al 2005 ;Green et al 2011 ;Squirrell et al 2006 ) .…”

Translational Control in the Caenorhabditis elegans Germ Line

“…Although the UGU core sequence of an FBE is essential for all tested PUF proteins to bind RNAs with high af fi nity, additional fl anking nucleotides add to their selectivity and mode of recognition. For example, PUF-8 preferentially binds to an eight nucleotide long motif with the consensus sequence of 5 ¢ -UGU ANAUA-3 ¢ , whereas FBF proteins prefer a nine nucleotide long 5 ¢ -UGU RNNAUA-3 ¢ (R, purine; N, any base) (Boag et al 2005 ) are also known as cold-shock domain proteins. The proteins LARP-1 (Nykamp et al 2008 ;Zanin et al 2010 ) and CAR-1 (Squirrell et al 2006 ;Audhya et al 2005 ;Boag et al 2005 ) contain an La-type and LSM-like RNA-binding motif, respectively GC germ cell.…”