CRYO-EM VISUALIZATION OF AN EXPOSED RGD EPITOPE ON ADENOVIRUS THAT ESCAPES ANTIBODY NEUTRALIZATION

Interaction of the adenovirus penton base protein with av integrins pr omotes virus entry into host cells, The location of the integrin bindi ng sequence Arg-Gly-Asp (RGD) on human type 2 adenovirus (Ad2) was vis ualized by cryo-electron microscopy (cryo-EM) and image reconstruction using a mAb (DAV-1) which recognizes a linear epitope, IRGDTFATR, The sites for DAV-1 binding corresponded to the weak density above each o f the five 22 Angstrom protrusions on the adenovirus penton base prote in. Modeling of a Fab fragment crystal structure into the adenovirus-F ab cryo-EM density indicated a large amplitude of motion for the Fab a nd the RGD epitope. An unexpected finding was that Fab fragments, but not IgG antibody molecules, inhibited adenovirus infection, Steric hin drance from the adenovirus fiber and a few bound IgG molecules, as wel l as epitope mobility, most likely prevent binding of IgG antibodies t o all five RGD sites on the penton base protein within the intact viru s, These studies indicate that the structure of the adenovirus particl e facilitates interaction with cell integrins, whilst restricting bind ing of potentially neutralizing antibodies.