Ubiquitination of mRNA cycling sequence binding protein from Leishmania donovani (LdCSBP) modulates the RNA endonuclease activity of its Smr domain.

In trypanosomatid parasites, an octanucleotide sequence (C/A)AUAGAA(G/A) in the UTRs primarily determines the stability of S-phase specific mRNAs. A multi-domain protein LdCSBP from Leishmania donovani interacts with the UTR of an S-phase RNA containing the octanucleotide sequence through its unique CCCH-type Zn-finger motifs. Interestingly, the RNA binding protein contains a previously characterized DNA endonuclease domain - Smr. It has been demonstrated here that the LdCSBP Smr domain independently possesses both DNA and RNA endonuclease activities, but the full-length LdCSBP exhibits only riboendonuclease activity. Moreover, LdCSBP protein has been shown to be ubiquitinated, resulting in the down-regulation of its riboendonuclease activity. In conclusion, the results described here suggest a novel regulatory mechanism of mRNA degradation through ubiquitination in eukaryotes.