Identification and characterization of a PRP5 / DDX46 ‐like DEAD ‐box helicase with noncanonical

The DEAD‐box proteins of the superfamily II (SF2) RNA/DNA helicases are typically found to be ATP‐dependent RNA binding proteins and RNA‐dependent ATPases; however, they have poor, nonprocessive, RNA unwinding activity. Moreover, cells contain multiple variants that are associated with specific cellular processes, and they are generally not interchangeable. Thus, they can be thought of as ATP‐dependent switches to control the progression and directionality of cellular metabolism and catabolism. We have characterized a DEAD‐box protein (LINF08) from the trypanosomatid parasite Leishmania infantum that has homology to the spliceosome protein DDX46/Prp5. It is expressed in the nucleus of the flagellated promastigotes and the immobile axenic amastigotes. It has very high intrinsic ATPase activity that is independent of added nucleic acids, and it is activated both by the usual bound magnesium cation and by calcium, which is a cation that typically does not directly participate in catalysis. DDX46/Prp5 in other eukaryotes is involved in the early proofreading steps of spliceosome assembly to resolve structural aberrations between the U2 small nuclear RNA (snRNA) and the splice junction. Although trypanosomes typically lack introns, all of the mRNAs are transcribed as polycistronic RNAs that are processed into individual mRNAs involving a process known as SL‐dependent trans splicing. Trypanosomes retain the spliceosome machinery but the snRNAs are in an abbreviated form. Thus, although LINF08 appears to be a DDX46/Prp5 homolog, it has probably evolved for the unusual splicing machinery and highly variable environment of the parasite.