It is generally assumed that the amino acids that exist in all homologous enzymes correspond to residues that participate in catalysis, or that are essential for folding and stability. Although this holds for catalytic residues, the function of conserved noncatalytic residues is not clear. It is not known if such residues are of equal importance and have the same role in different homologous enzymes. In humans, the E104D mutation in triosephosphate isomerase (TIM) is the most frequent mutation in the autosomal diseases named "TPI deficiencies." We explored if the E104D mutation has the same impact in TIMs from four different organisms (Homo sapiens, Giardia lamblia, Trypanosoma cruzi, and T. brucei). The catalytic properties were not significantly affected by the mutation, but it affected the rate and extent of formation of active dimers from unfolded monomers differently. Scanning calorimetry experiments indicated that the mutation was in all cases destabilizing, but the mutation effect on rates of irreversible denaturation and transition-state energetics were drastically dependent on the TIM background. For instance, the E104D mutation produce changes in activation energy ranging from 430 kJ mol(-1) in HsTIM to -78 kJ mol(-1) in TcTIM. Thus, in TIM the role of a conserved noncatalytic residue is drastically dependent on its molecular background. Accordingly, it would seem that because each protein has a particular sequence, and a distinctive set of amino acid interactions, it should be regarded as a unique entity that has evolved for function and stability in the organisms to which it belongs.

SUMMARYWith a reassortant from a cross of human rotavirus DS-1 (serotype 2) and OSU (serotype 5) it was determined that the OSU major neutralization glycoprotein antigen (VP7) was encoded by gene segment 9. A full-sized cloned cDNA copy of the OSU gene 9 was produced and sequenced. Hybridization of such labelled cDNA with the corresponding segment of a reassortant DS-1 x OSU virus confirmed the coding assignment. Comparison of the deduced amino acid sequence of the VP7 of OSU with those previously determined for five other rotavirus strains, representing four distinct serotypes, revealed some hydrophilic regions that exhibited significant homology and other hydrophilic domains with greater amino acid divergence. In one of the latter hydrophilic domains each of the five serotypes had a distinct amino acid substitution at residue 146, suggesting that it may be involved in serotype specificity.