Subassemblies and Asymmetry in Assembly of Herpes Simplex Virus

ABSTRACT The herpes simplex virus 1 (HSV-1) capsid is a massive particle (~200 MDa; 1,250-Å diameter) with T16 icosahe-dral symmetry. It initially assembles as a procapsid with ~4,000 protein subunits of 11 different kinds. The procapsid undergoes major changes in structure and composition as it matures, a process driven by proteolysis and expulsion of the internal scaffold-ing protein. Assembly also relies on an external scaffolding protein, the triplex, an 2 heterotrimer that coordinates neighbor-ing capsomers in the procapsid and becomes a stabilizing clamp in the mature capsid. To investigate the mechanisms that regu-late its assembly, we developed a novel isolation procedure for the metastable procapsid and collected a large set of cryo-electron microscopy data. In addition to procapsids, these preparations contain maturation intermediates, which were distinguished by classifying the images and calculating a three-dimensional reconstruction for each class. Appraisal of the procapsid structure led to a new model for assembly; in it, the protomer (assembly unit) consists of one triplex, surrounded by three major capsid pro-tein (MCP) subunits. The model exploits the triplexes ’ departure from 3-fold symmetry to explain the highly skewed MCP hex-amers, the triplex orientations at each 3-fold site, and the T16 architecture. These observations also yielded new insights into maturation. IMPORTANCE This paper addresses the molecular mechanisms that govern the self-assembly of large, structurally complex, mac-romolecular particles, such as the capsids of double-stranded DNA viruses. Although they may consist of thousands of protein subunits of many different kinds, their assembly is precise, ranking them among the largest entities in the biosphere whos