Cleavage and Structural Transitions during Maturation of Staphylococcus aureus Bacteriophage 80???± and SaPI1 Capsids

摘要

In the tailed bacteriophages, DNA is packaged into spherical procapsids, leading to expansion into angular, thin-walled mature capsids. In many cases, this maturation is accompanied by cleavage of the major capsid protein (CP) and other capsid-associated proteins, including the scaffolding protein (SP) that serves as a chaperone for the assembly process. Staphylococcus aureus bacteriophage 80???± is capable of high frequency mobilization of mobile genetic elements called S. aureus pathogenicity islands (SaPIs), such as SaPI1. SaPI1 redirects the assembly pathway of 80???± to form capsids that are smaller than those normally made by the phage alone. Both CP and SP of 80???± are N-terminally processed by a host-encoded protease, Prp. We have analyzed phage mutants that express pre-cleaved or uncleavable versions of CP or SP, and show that the N-terminal sequence in SP is absolutely required for assembly, but does not need to be cleaved in order to produce viable capsids. Mutants with pre-cleaved or uncleavable CP display normal viability. We have used cryo-EM to solve the structures of mature capsids from an 80???± mutant expressing uncleavable CP, and from wildtype SaPI1. Comparisons with structures of 80???± and SaPI1 procapsids show that capsid maturation involves major conformational changes in CP, consistent with a release of the CP N-arm by SP. The hexamers reorganize during maturation to accommodate the different environments in the 80???± and SaPI1 capsids.