The messenger RNA decapping and recapping pathway inTrypanosoma

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-06, Vol.112 (22), p.6967-6972
• Main Authors: Ignatochkina, Anna V., Takagi, Yuko, Liu, Yancheng, Nagata, Kyosuke, Ho, C. Kiong
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490

The 5′ terminus of trypanosome mRNA is protected by a hypermethylated cap 4 derived from spliced leader (SL) RNA.Trypanosoma bruceinuclear capping enzyme with cap guanylyltransferase and methyltransferase activities (TbCgm1) modifies the 5′-diphosphate RNA (ppRNA) end to generate an m7G SL RNA cap. Here we show thatT. bruceicytoplasmic capping enzyme (TbCe1) is a bifunctional 5′-RNA kinase and guanylyltransferase that transfers a γ-phosphate from ATP to pRNA to form ppRNA, which is then capped by transfer of GMP from GTP to the RNA β-phosphate. A Walker A-box motif in the N-terminal domain is essential for the RNA kinase activity and is targeted preferentially to a SL RNA sequence with a 5′-terminal methylated nucleoside. Silencing of TbCe1 leads to accumulation of uncapped mRNAs, consistent with selective capping of mRNA that has undergonetrans-splicing and decapping. We identifyT. bruceimRNA decapping enzyme (TbDcp2) that cleaves m7GDP from capped RNA to generate pRNA, a substrate for TbCe1. TbDcp2 can also remove GDP from unmethylated capped RNA but is less active at a mature cap 4 end and thus may function in RNA cap quality surveillance. Our results establish the enzymology and relevant protein catalysts of a cytoplasmic recapping pathway that has broad implications for the functional reactivation of processed mRNA ends.