Adenovirus penton base protein, molecular model. This protein molecule is a subunit called a penton, forming the vertices of the capsid of this adenovirus. This penton base is a polymer consisting of 523 amino acids. The penton is part of the capsid structure that attaches to a cell membrane and aids transmission of the viral genetic material into the host cell. This penton base structure was modelled for human adenovirus 2 (hAd2). There are many types of adenovirus. They commonly cause infections of the upper respiratory tract, causing cold-like symptoms, particularly in children.