The human adenoviruses are responsible for a number of respiratory, gastroenteric and ocular infections. They also make good candidates as delivery vehicles for gene therapy applications and are therefore of intense interest among various biotechnology companies. They form icosahedral particles with 240 copies of the trimeric hexon protein arranged on the planes of the icosahedron and a penton complex at each of the twelve vertices. The penton comprises a pentameric base, implicated in virus internalisation, and a protruding trimeric fibre, responsible for the attachment of the virus to the receptor. The fibres are homo-trimeric proteins containing an amino terminal penton base attachment domain, a long thin central shaft, and a carboxy terminal cell attachment or head domain [1]. The primary sequence of the fibre shaft consists of a 15 residue pseudo-repeat unit and there are 22 repeats in both the serotype 2 and 5 of the human adenovirus. The shaft gives the virus a long reach to search for CAR (Coxsackievirus and Adenovirus Receptor) receptors on cell surfaces. The CAR is lodged in the outer membrane of most human cells and has an antenna-like projection that extends outside the cell. Although CAR's function in healthy cells is still unknown, it has been recently shown that the receptor recognises the head domain of the adenovirus. When the virus approaches the cell, the fibre head binds to CAR as a first step in gaining access to the cell. It is also the link that will have to be broken if the adenovirus is to become a useful therapeutic tool, since it will be required to bind not to the majority, but only to the target cells.

It has been possible to crystallise a recombinant protein comprising the four distal repeats of the adenovirus type 2 shaft plus the receptor-binding head domain. Using beamline ID14/3, data to a resolution of 2.4 Å was collected at 100 K and the structure solved by molecular replacement using the Ad2 fibre head structure alone [2].

The shaft structure reveals a new fold, termed the "triple ß-spiral", as shown in Figure 9. This is a regular, highly cross-linked structure which together with a high proportion of buried surface (one third of the solvent accessible surface of a shaft monomer is buried on trimer formation) gives a high rigidity and stability to the shaft. The average diameter of the shaft is some 15 Å, although surface loops give a maximum diameter of 22 Å and a model of the full length fibre, based on the four repeats in the current structure, indicates a total shaft length of some 300 Å. The stability and morphology of the adenovirus shaft have led to its use as a model for synthetic fibre design. As indicated in Figure 10, the three fibres meet at the fibre head and van Raaij and Cusack have been able to identify grooves between adjacent subunits where CAR molecules bind. This, in turn, has enabled them to propose mutations of amino acids which will alter or abolish the binding of CAR without interfering with the overall structure of the fibre. Such proposals are being investigated by genetic engineering enterprises.

[1] M.J. van Raaij, N. Louis, J. Chroboczek, S. Cusack, Virology, 70, 7071-8 (1996).
[2] M.J. van Raaij, A. Mitraki, G. Lavigne, S. Cusack, Nature, 401, 935-8 (1999).

M.J. van Raaij (a), A. Mitraki, (b), J. Chroboczek (b), S. Cusack (a).

(a) EMBL Grenoble Outstation (France)
(b) IBS (CEA-CNRS), Grenoble (France)