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, and in endocytosis of virions. As the virus enters the host cell, penton proteins are shed concomitant with virion acidification in the endosome (By similarity). 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 (By similarity). Located at each vertex of the virion. Expressed in the late phase of the viral replicative cycle. 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 (By similarity). Belongs to the adenoviridae penton family.