Mechanism of scaffolding-directed virus assembly suggested by comparison of scaffolding-containing and scaffolding-lacking P22 procapsids

Assembly of certain classes of bacterial and animal viruses requires the tr ansient presence of molecules known as scaffolding proteins, which are esse ntial for the assembly of the precursor procapsid, To assemble a procapsid of the proper size, each viral coat subunit must adopt the correct quasiequ ivalent conformation from several possible choices, depending upon the T nu mber of the capsid. In the absence of scaffolding protein, the viral coat p roteins form aberrantly shaped and incorrectly sized capsids that cannot pa ckage DNA. Although scaffolding proteins do not form icosahedral cores with in procapsids, an icosahedrally ordered coat/scaffolding interaction could explain how scaffolding can cause conformational differences between coat s ubunits, To identify the interaction sites of scaffolding protein with the bacteriophage P22 coat protein lattice, we have determined electron cryomic roscopy structures of scaffolding-containing and scaffolding-lacking procap sids. The resulting difference maps suggest specific interactions of scaffo lding protein with only four of the seven quasiequivalent coat protein conf ormations in the T = 7 P22 procapsid lattice, supporting the idea that the conformational switching of a coat subunit is regulated by the type of inte ractions it undergoes with the scaffolding protein. Based on these results, we propose a model for P22 procapsid assembly that involves alternating st eps in which first coat, then scaffolding subunits form self-interactions t hat promote the addition of the other protein. Together, the coat and scaff olding provide overlapping sets of binding interactions that drive the form ation of the procapsid.