Biting the hand that feeds you: sialidases that bind and hydrolyse sialic acid

Garry Taylor¹, Helen Connaris¹, Ibrahim Moustafa¹, Simon Newstead¹, Margaret Taylor¹, Viateslav Zaitsev¹, Toru Takimoto², Allen Portner²

¹Centre for Biomolecular Sciences, University of St Andrews, St Andrews, Fife KY16 9ST, Scotland

²Department of Infectious Diseases, St Jude Children’s Research Hospital, Memphis, TN, USA

The hemagglutinin-neuraminidase (HN) glycoprotein found on the surface of paramyxoviruses has three functions: it binds to sialic acid receptors, it hydrolyses the removal of sialic acid, and it interacts with the other surface glycoprotein, the fusion (F) protein, to promote fusion of the viral and host cell membranes. For many years the sialic acid binding and hydrolysis functions were thought to reside in two distinct domains on the protein. Our recent determination of the crystal structure of HN from Newcastle disease virus, shown below, (Crennell et al., 2000), suggested that, remarkably, the two functions reside in a single sialic acid recognition site. The site switches function through a concerted movement of amino acids that are conserved across the paramyxovirus family. We have carried out site-directed mutagenesis which support the single switchable site (Connaris et al., 2002). Further mutagenesis studies will be presented that suggest which parts of HN are involved in the promotion of fusion. The single site provides an attractive basis for the development of inhibitors that can stop both the initial infection and the spread of the virus. How binding of sialic acid to HN triggers fusion is unknown, but some ideas from a molecular dynamics simulation will be explored.

The sialidase of Vibrio cholerae (VCNA) has a catalytic domain flanked by two lectin-like domains (Crennell et al., 1994). It has long been known that VCNA can bind to the surface of cells and remain catalytically active. Recently we have discovered that one of the lectin domains recognises sialic acid. The nature of the recognition is quite different to other sialic acid recognising proteins, and will be presented together with the results of calorimetry used to quantify the interaction.

HN and VCNA therefore show two ways in which sialic acid binding and hydrolysis can be combined in one protein.

Crennell, S.J., Garman, E.F., Laver, W.G., Vimr, E.R. and Taylor, G.L. Structure 2, 535-544 (1994).

Crennell, S.J., Takimoto, T., Portner, A. and Taylor, G. (2000) Nature Struc. Biol. 7, 1068-1074

Connaris, H., Takimoto, T., Russell, R., Crennell, S., Moustafa, I., Portner, A., and Taylor, G. (2002) J. Virol 76, 1816-1824.