A novel essential RNA-binding protein complex in Trypanosoma brucei

ZC3H5 is an essential cytoplasmic trypanosome protein with a single Cx7Cx5Cx3H zinc finger domain. We here show that ZC3H5 forms a complex with three other proteins, encoded by genes Tb927.11.4900, Tb927.8.1500 and Tb927.7.3040. ZC3H5 interacts directly with Tb927.11.4900, which in turn interacts with Tb927.7.3040. Tb927.11.4900 has a circularly permuted GTPase domain, which is required for the Tb927.7.3040 interaction. RNA immunoprecipitation revealed that ZC3H5 is preferentially associated with poorly translated, low-stability mRNAs, the 5-untranslated regions and coding regions of which are enriched in the motif (U/A)UAG(U/A). Tethering of ZC3H5, or other complex components, to a reporter repressed its expression. However, depletion of ZC3H5 in vivo did not increase the abundance of ZC3H5-bound mRNAs: instead, counter-intuitively, there were very minor decreases in a few targets, and marked increases in the abundances of very stable mRNAs encoding ribosomal proteins. Depletion also resulted in an increase in monosomes at the expense of large polysomes, and appearance of "halfmer" disomes containing two 80S subunits and one 40S subunit. We speculate that the ZC3H5 complex might be implicated in quality control during the translation of sub-optimal open reading frames; complex assembly might be regulated by GTP hydrolysis and GTP-GDP exchange.