Major capsid protein that self-associates to form penton base pentamers, each in the shape of a pentagon, situated at the 12 vertices of the pseudo T=25 capsid. Involved in virus secondary attachment to host cell after initial attachment by the fiber protein. Binds host integrin heterodimer ITGAV-ITGB5 (alphaV-beta5) thereby triggering clathrin-mediated endocytosis of virions. Mediates initial virus attachment to CXADR-negative cells. Binding to integrins ITGAV-ITGB5 also seems to induce macropinocytosis uptake of the virus. As the virus enters the host cell, penton proteins are shed concomitant with virion acidification in the endosome. Interacts (via the cell attachment site RGD) with host heterodimer ITGAV-ITGB5; this interaction promotes virus internalization. Interacts with host WWP1 and WWP2. Interacts with the fiber protein (via N-terminal tail region). Interacts with the capsid vertex protein; this interaction binds the penton base to neighboring peripentonal hexons. Located at each vertex of the virion. Present in 60 copies per virion. Expressed in the late phase of the viral replicative cycle. The cell attachment RGD motif is exposed at the virion surface and is involved in binding to the integrin heterodimer ITGAV-ITGB5. All late proteins expressed from the major late promoter are produced by alternative splicing and alternative polyadenylation of the same gene giving rise to non-overlapping ORFs. A leader sequence is present in the N-terminus of all these mRNAs and is recognized by the viral shutoff protein to provide expression although conventional translation via ribosome scanning from the cap has been shut off in the host cell. Belongs to the adenoviridae penton family.