Discussion

5' to 3' decay is the major mRNA degradation pathway in many organisms, including trypanosomes. It starts with cap-removal by the decapping enzyme DCP2 and finishes with 5'-3' decay by the exoribonuclease Xrn1 (XRNA in trypanosomes). Kinetoplastids have decapping activity, but are the only eukaryotes with no obvious orthologue to DCP2. Here we provide evidence for an ApaH-like phosphatase (ALPH1) being the trypanosome decapping enzyme; this protein had been identified as a novel RNA granule component by our recent purification of trypanosome stress granules. ApaH is a subgroup of bacterial phosphatases with diadenosine tetraphosphatase activity: an activity reminiscent of mRNA decapping.  I found striking similarities between XRNA and ALPH1: RNAi depletion of both proteins is lethal and causes stabilisation of total mRNAs. Moreover, ALPH1 and XRNA, but no other proteins, co-localise to a special granule at the posterior pole of the cell. I have recently developed a novel fluorescent tool for the detection of mRNA decay intermediates on subcellular resolution. RNAi depletion of XRNA causes an increase in both intact mRNAs and in 5'-3' decay intermediates, consistent with a function in mRNA degradation. In contrast, RNAi depletion of ALPH1 causes an increase in intact mRNAs only, consistent with a function in the initiation step of 5'-3' mRNA decay. Together, these data provide strong evidence for ALPH1 acting upstream of XRNA in the 5'-3' decay pathway, possibly in decapping.