PROTEIN CHAINMAIL - CATENATED PROTEIN IN VIRAL CAPSIDS

The capsid shells of bacteriophage HK97 and several other phages conta in polypeptides that are covalently linked into complexes so large tha t they do not enter polyacrylamide gels after denaturation. The enormo us apparent size of these protein complexes in HK97 derives from a nov el protein topology. HK97 subunits cross-link via isopeptide bonds int o oligomers that are closed rings of five or six members. However, pol ypeptides from neighboring pentamer and hexamer rings intertwine befor e the covalent cross-links form. As a result, adjacent protein rings c atenate into a network similar to chainmail armor. In vitro linking an d unlinking experiments provide strong support for the chainmail model , which explains the unusual properties of these bacteriophages and ma y apply to other macromolecular structures.