Structural Studies of Phage Lysis Proteins and Their Targets

Bacteriophages (phages) are viruses that infect bacteria. The phages that are described by this dissertation encompass 2 classes, double-stranded DNA phages and single-stranded RNA phages. While both of these phages infect similar bacteria, they have adopted different mechanisms to lyse, or destroy, the cell in order to release phage progeny. dsDNA phages have large genomes (> 20 kb) and use multiple lysis proteins (holin, endolysin, and spanin complex) to lyse the cell. ssRNA phages, on the other hand, have small genomes (< 6 kb) and only encode one lysis protein.

The two X-ray crystallography projects outlined here deal with the phage proteins involved in these lysis mechanisms.

The project described in the first study deals with the holin (T) and the antiholin (RI) of the ds-DNA phage T4, the major players of the lysis inhibition (LIN) phenomenon. Crystal structures of the holin and of the holin-antiholin complex are presented. The structures provide new molecular level insights into the phenomenon of LIN in bacteriophage T4 and the T-even phages in general.

The second investigation describes ongoing efforts at structural characterization of A2, the maturation protein of the ssRNA bacteriophage Qbeta that inhibits E. coli MurA. In addition, the structure of Bacillus subtilis MurA, which is not recognized by A2, is presented. The crystal structure of B. subtilis MurA, the first structure of MurA from a Gram-positive organism, allows for a direct comparison of Gram-positive and Gram-negative homologs and for identification of any significant structural differences. The more flexible catalytic loop of B. subtilis MurA protrudes farther out compared to the loop of E. coli MurA and creates enough hindrance to prevent A2 from establishing secure contact points.