The ubiquitin-binding RPN-10 protein serves as a ubiquitin receptor that delivers client proteins to the 26S proteasome. Although ubiquitin recognition is an essential step for proteasomal destruction, deletion of the rpn-10 gene in yeast does not influence viability, indicating redundancy of the substrate delivery pathway. However, their specificity and biological relevance in higher eukaryotes is still enigmatic. We report herein that knockdown of the rpn-10 gene, but not any other proteasome subunit genes, sexually transforms hermaphrodites to females by eliminating hermaphrodite spermatogenesis in Caenorhabditis elegans. The feminization phenotype induced by deletion of the rpn-10 gene was rescued by knockdown of tra-2, one of sexual fate decision genes promoting female development, and its downstream target tra-1, indicating that the TRA-2-mediated sex determination pathway is crucial for the ⌬rpn-10-induced sterile phenotype. Intriguingly, we found that co-knockdown of rpn-10 and functionally related ubiquitin ligase ufd-2 overcomes the germline-musculinizing effect of fem-3(gf). Furthermore, TRA-2 proteins accumulated in rpn-10-defective worms. Our results show that the RPN-10 -mediated ubiquitin pathway is indispensable for control of the TRA-2-mediated sexdetermining pathway.
INTRODUCTIONUbiquitin is a covalent modifier that produces a polyubiquitin chain functioning as a degradation signal (Hershko and Ciechanover, 1998;Pickart, 2001). Recruitment of polyubiquitinated proteins to the degradation machinery is a key step in the selective degradation of various cellular proteins (Pickart, 1998;Voges et al., 1999;Madura, 2004). Polyubiquitin chains with lengths of at least four ubiquitin moieties can be recognized and degraded by the 26S proteasome, a eukaryotic ATP-dependent protease complex (Voges et al., 1999;Thrower et al., 2000). The 26S proteasome is composed of the catalytic 20S proteasome and the regulatory PA700 complex; the later complex consists of six ATPase subunits (RPT-1-6) and multiple non-ATPase subunits , each ranging in size from 11 to 110 kDa (Tanaka, 1998).Previous studies have shown that the RPN-10 subunit of PA700, originally called S5a, can bind to a polyubiquitin chain linked to proteins in vitro and in vivo, and it is thought to play a role as a ubiquitin receptor of the 26S proteasome (Deveraux et al., 1994;Ferrell et al., 1996;van Nocker et al., 1996;Kawahara et al., 2000;Wilkinson et al., 2001;Elsasser et al., 2004;Verma et al., 2004). Figure 1 shows an abbreviated model that highlights the RPN-10 ubiquitinrecognition subunits in 26S proteasomes. The RPN-10 has at least two distinct domains: one domain is the 60-residue N-terminal called the VWA domain, which was reported to be involved in the integration of the 26S complex Verma et al., 2004), and the other domain is the C-terminal region containing two independent polyubiquitin-binding sites, named ubiquitin interacting motif (UIM)1 and UIM2 (Young et al., 1998;Hofmann and Falquet, 2001). Although it was thought that ubiquiti...
