Thermal stability of the hemagglutinin-neuraminidase from Sendai virus evidences two folding domains.

The domain structure of hemagglutinin-neuraminidase from Sendai virus (cHN) was investigated by studying the thermal stability in the 20-100 degrees C range. Differential scanning calorimetry evidences two conformational transitions. The first transition is apparently a reversible two-state process, with Tm 48.3 degrees C, and is shifted to 50.1 degrees C in the presence of the substrate analogue 2,3-dehydro-2-deoxy-N-acetyl neuraminic acid, meaning that the substrate binding domain is involved in the transition. The second transition, with apparent Tm 53.2 degrees C, is accompanied by irreversible loss of enzymatic activity of the protein, and the presence of the substrate analogue does not affect the Tm. The data indicate that cHN is composed of two independent folding domains, and that only one domain is involved in the binding of the substrate. Our results suggest that the paramyxovirus neuraminidases have the folding properties of a two-domain protein.