Procyclic Trypanosoma brucei expresses separate sialidase and trans-sialidase enzymes on its surface membrane

GEORGINA N. MONTAGNA; JOHN DONELSON; ALBERTO C.C. FRASCH

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Lugar: Bethesda, Maryland; Año: 2006

0021-9258

The procyclic stage of Trypanosoma brucei in the insect vectorexpresses a surface-bound trans-sialidase (TbTS) that transferssialic acid from glycoconjugates in the environment to glycosylphosphatidylinositol-anchored proteins on its surfacemembrane. RNA interference against TbTS abolished transsialidaseactivity in procyclic cells but did not diminish sialidaseactivity, suggesting the presence of a separate sialidase enzymefor hydrolyzing sialic acid. A search of the T. brucei genomesequence revealed seven other putative genes encoding proteinswith varying similarity to TbTS. RNA interference directedagainst one of these proteins, TbSA C, greatly decreased thesialidase activity but had no effect on trans-sialidase activity.The deduced amino acid sequence of TbSA C shares only 40%identity with TbTS but conserves most of the relevant residuesrequired for catalysis. However, the sialidase has a tryptophansubstitution for a tyrosine at position 170 that is crucial in bindingthe terminal galactose that accepts the transferred sialicacid. When this same tryptophan substitution in the sialidasewas placed into the recombinant trans-sialidase, the mutantenzyme lost almost all of its trans-sialidase activity andincreased its sialidase activity, further confirming that the geneand protein identified correspond to the parasite sialidase.Thus, in contrast to all other trypanosomes analyzed to date thatexpress either a trans-sialidase or a sialidase but not both,T. brucei expresses these two enzymatic activities in two separateproteins. These results suggest that African trypanosomescould regulate the amount of critical sialic acid residues on theirsurface by