Identification of the HIT-45 protein from Trypanosoma brucei as an FHIT protein/dinucleoside triphosphatase: substrate specificity studies on the recombinant and endogenous proteins.

A new member of the FHIT protein family, designated HIT-45, has been identified in the African trypanosome Trypanosoma brucei. Recombinant HIT-45 proteins were purified from trypanosomal and bacterial protein expression systems and analyzed for substrate specificity using various dinucleoside polyphosphates, including those that contain the 5'-mRNA cap, i.e., m(7)GMP. This enzyme exhibited typical dinucleoside triphosphatase activity (EC 3.6.1.29), having its highest specificity for diadenosine triphosphate (ApppA). However, the trypanosome enzyme contains a unique amino-terminal extension, and hydrolysis of cap dinucleotides with monomethylated guanosine or dimethylated guanosine always yielded m(7)GMP (or m(2,7)GMP) as one of the reaction products. Interestingly, m(7)Gpppm(3)(N6, N6, 2'O)A was preferred among the methylated substrates. This hypermethylated dinucleotide is unique to trypanosomes and may be an intermediate in the decay of cap 4, i.e., m(7)Gpppm(3)(N6, N6, 2'O)Apm(2'O)Apm(2'O)Cpm(2)(N3, 2'O)U, that occurs in these organisms.