Structural component of the virion that acts as a cement protein on the capsid exterior which mediates the interactions between the hexons, including the peripentonal hexons, and reaches all the way to the penton vertices. Two hexon linking proteins IIIa, one from each facet, stabilize the unique edge interface between a pair of facets. As the virus enters the host cell, hexon linking proteins IIIa are shed concomitant with virion acidification in the endosome. During virus assembly, seems to play a role in the serotype specificity of the packaging of viral DNA via its interaction with packaging protein 3. Interacts with hexon proteins; this interaction tethers the peripentonal hexons to hexons situated in the facet. Interacts with the penton protein (via N-terminus). Interacts with packaging protein 3; this interaction is required to promote correct genome packaging. Surrounds the border of each facet on the capsid exterior. Present in around 60 copies per virion. Expressed in the late phase of the viral replicative cycle. Cleaved near the C-terminus by the viral protease during virion maturation to form the mature protein. 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 hexon-linking protein IIIa family.