Biology Department Seminar

Adenovirus infection is initiated by the binding of viral fiber proteins to the coxsackie-adenovirus receptor (CAR) on the host cell plasma membrane. Assembly of the homotrimeric fiber protein is nucleated by trimerization of its C-terminal knob domain, which itself can assemble into trimers when expressed as a recombinant protein fragment in Escherichia coli. We found that plasmids directing expression of the adenovirus-2 knob domain could not be established in thi- strains of E. coli and that knob protein produced in thi+ strains was tightly asssociated with thiamine diphosphate (ThDP). Further studies led us to propose a model where ThDP functions as a chemical chaperone to promote knob trimer assembly, possibly by stabilizing interfacial surfaces that are exposed on assembly intermediates. These results have implications for mechanisms of oligomeric protein assembly in general, and suggest a novel strategy for anti-viral therapy. The CAR-binding activity of fiber is associated solely with the knob domain, which has 3 equivalent, high affinity binding sites for CAR. We injected mice with recombinant knob protein to determine the ability of these trivalent 60 kD trimers to target CAR in vivo. Knob was taken up and degraded in liver by a CAR-dependent pathway, indicating a novel function for this receptor distinct from its known role as a homophilic adhesion molecule in the junctional complex of epithelial cells. Knob also induced secretion of prostatic proteins, indicating that the protein can bind to CAR in the prostate epithelium and trigger a physiological response. These results are encouraging in terms of developing knob-based reagents for prostate imaging and for other applications that require specific receptor targeting in vivo.