A structure of adenovirus type 12 (HAdV12) complexed with a soluble form of integrin alphavbeta5 was determined by cryo-electron microscopy (cryoEM) image reconstruction. Subnanometer resolution (8 A) was achieved for the icosahedral capsid with moderate resolution (27 A) for integrin density above each penton base. Modeling with alphavbeta3 and alpha(IIb)beta3 crystal structures indicates that a maximum of four integrins fit over the pentameric penton base. The close spacing (approximately 60 A) of the RGD protrusions on penton base precludes integrin binding in the same orientation to neighboring RGD sites. Flexible penton-base RGD loops and incoherent averaging of bound integrin molecules explain the moderate resolution observed for the integrin density. A model with four integrins bound to a penton base suggests that integrin might extend one RGD-loop in the direction that could induce a conformational change in the penton base involving clockwise untwisting of the pentamer. A global conformational change in penton base could be one step on the way to the release of Ad vertex proteins during cell entry. Comparison of the cryoEM structure with bent and extended models for the integrin ectodomain reveals that integrin adopts an extended conformation when bound to the Ad penton base, a multivalent viral ligand. These findings shed further light on the structural basis of integrin binding to biologically relevant ligands, as well as on the molecular events leading to HAdV cell entry.