Abstract
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.