Insights into a viral motor: the structure of the HK97 packaging termination assembly

Abstract

ABSTRACTDouble-stranded DNA viruses utilise machinery, made of terminase proteins, to package viral DNA into the capsid. Forcosbacteriophage, a defined signal, recognised by small terminase, flanks each genome unit. Here we present the first structural data for acosvirus DNA packaging motor, assembled from the bacteriophage HK97 terminase proteins, procapsids encompassing the portal protein, and DNA containing acossite. The cryo-EM structure is consistent with the packaging termination state adopted after DNA cleavage, with DNA density within the large terminase assembly ending abruptly at the portal protein entrance. Retention of the large terminase complex after cleavage of the short DNA substrate suggests that motor dissociation from the capsid requires headful pressure, in common withpacviruses. Interestingly, the clip domain of the 12-subunit portal protein does not adhere to C12symmetry, indicating asymmetry induced by binding of the large terminase/DNA. The motor assembly is also highly asymmetric, showing a ring of 5 large terminase monomers, tilted against the portal. Variable degrees of extension between N- and C-terminal domains of individual subunits suggest a mechanism of DNA translocation driven by inter-domain contraction and relaxation.