Structures of three actinobacteriophage capsids: Roles of symmetry and accessory proteins

Mycobacterium tuberculosis and abscessus are major human pathogens that are part of the Actinobacteria phylum. Increasing multiple drug resistance in these bacteria has led to a renewed interest in using viruses that infect these bacteria for therapy. In order to understand these viruses, a course-based undergraduate research experience (CURE) program run by SEA-PHAGES at the University of Pittsburgh and HHMI has isolated, sequenced, and annotated over 3000 actinobacteriophages (viruses that infect Actinobacteria). Little work has been done to investigate the structural diversity of these phage, all of which are thought to use a common protein fold, the HK97-fold, in their major capsid protein. Here we describe the structure of three actinobacteriophage capsids isolated by students that infect Mycobacterium smegmatis. The capsid structures were resolved to approximately 6 angstroms, which allowed confirmation that each phage uses the HK97-fold to form their capsid. One phage, Rosebush, has a novel variation of the HK97-fold. Four novel accessory proteins, that form the capsid head along with the major capsid protein, were identified that show limited or no homology to known proteins. The genes that encode the proteins were identified using SDS-PAGE and mass spectrometry. Bioinformatic analysis of the accessory proteins suggest they are used in many actinobacteriophage capsids.