Topologically linked protein rings in the bacteriophage HK97 capsid

The crystal structure of the double-stranded DNA bacteriophage HK97 mature empty capsid was determined at 3.6 angstrom resolution. The 660 angstrom di ameter icosahedral particle contains 420 subunits with a new fold. The fina l capsid maturation step is an autocatalytic reaction that creates 420 isop eptide bonds between proteins. Each subunit is joined to two of its neighbo rs by Ligation of the side-chain Lysine 169 to asparagine 356. This generat es 12 pentameric and 60 hexameric rings of covalently joined subunits that loop through each other, creating protein chainmail: topologically linked p rotein catenanes arranged with icosahedral symmetry. Catenanes have not bee n previously observed in proteins and provide a stabilization mechanism for the very thin HK97 capsid.