Authors

Discussion

The 5′ ends of eukaryotic mRNAs are co-transcriptionally modified with a protective m⁷G cap structure. In the course of RNA turnover, this cap is removed by the major eukaryotic mRNA decay enzyme, the nudix hydrolase Dcp2, that is part of the decapping complex. Trypanosomes lack homologs to all proteins of the decapping complex but instead use the ApaH-like phosphatase TbALPH1: this is unique to Kinetoplastida. ApaH like phosphatases (ALPHs) are a bacterial-derived class of enzymes present in all eukaryotic super-groups, but absent in mammals.

TbALPH1 is essential for cultured trypanosome cells and consists of the catalytic domain and unique C- and N-terminal extensions. We found that ALPH1 has in vitro decapping activity in a wide range of conditions without cap-type preference and, surprisingly, even in the absence of its C- and N-terminal domains. We found no evidence for RNA binding activity, consistent with the absence of any known RNA binding domains and the fact that ALPH1 accepts cap analogues as a substrate too. The N-terminal domain is dispensable even in vivo, but needed for localization of ALPH1 to the posterior pole of the cell. BioID data suggest that the C-terminus is required for interaction with the 5´ to 3´ exoribonuclease XRNA, the enzyme that acts downstream of ALPH1.

ALPH1 is one of only two eukaryotic ApaH like phosphatases with a known function and the only mRNA decapping enzyme that does not belong to the nudix hydrolase family. A better understanding of its mechanism and regulation is essential and our data are an important contribution. Moreover, the absence of ApaH like phosphatases in mammals rises the possibility that ALPH1 can be exploited as a drug target and we are currently investigating this option.