Phosphoglucomutase is absent in Trypanosoma brucei and redundantly substituted by phosphomannomutase and phospho-N-acetylglucosamine mutase

Abstract

The enzymes phosphomannomutase (PMM), phospho-N-acetylglucosamine mutase (PAGM) and phosphoglucomutase (PGM) reversibly catalyze the transfer of phosphate between the C6 and C1 hydroxyl groups of mannose, N-acetylglucosamine and glucose, respectively. Although genes for a candidate phosphomannomutase and a phospho-N-acetylglucosamine mutase enzymes have been found in the Trypanosoma brucei genome there is, surprisingly, no candidate gene for phosphoglucomutase. The TbPMM and TbPAGM genes were cloned and expressed in Escherichia coli and the TbPMM enzyme was crystallized and its structure solved at 1.85Å resolution. Antibodies to the recombinant proteins localized endogenous TbPMM to glycosomes in the bloodstream form of the parasite while TbPAGM localized to both the cytosol and glycosomes. Both recombinant enzymes were able to interconvert glucose-phosphates, as well as acting on their own definitive substrates. Analysis of sugar nucleotide levels in parasites with TbPMM or TbPAGM knocked down by RNA interference (RNAi) suggests that, in vivo, PGM activity is catalyzed by both enzymes. This is the first example in any organism of PGM activity being completely replaced in this way and it explains why, uniquely, T. brucei has been able to lose its PGM gene. The RNAi data for TbPMM also showed that this is an essential gene for parasite growth.