UCSF

Tubulins are a superfamily of polymerizing GTPases, which are conserved from bacteria to man. Though they share a common fold and mode of longitudinal interaction (with the nucleotide at the interface), they have extremely diverse primary sequences and form diverse structures ranging from single protofilaments to rings to tubes. Tubulins rely on the energy from GTP to generate filament dynamics, which allows them to perform a diverse set of functions. Microtubules are dynamically unstable, switching from rapidly growing to shrinking states in order to search and capture chromosomes during mitosis. FtsZ filaments treadmill in vitro and organize the machinery for bacterial septation, though their in vivo dynamics are still not well understood. TubZ filaments treadmill to segregate large, low copy number plasmids in Bacillus. Although GTP is critical for polymerization and dynamics of tubulin superfamily members, the molecular requirement for GTP in polymerization and its role in dynamics are still not well understood.

The subject of this manuscript is the newly identified tubulin superfamily member PhuZ. PhuZ is encoded on the genomes of large Pseudomonas phage. This manuscript describes the early characterization of this tubulin, from its role in the phage lytic cycle to its structure and dynamics. A specific focus is on the structure and polymerization dynamics of PhuZ filaments. Understanding of how PhuZ polymerizes and uses GTP shed light on how tubulins use GTP in general, as well as unique characteristics of PhuZ filaments. We have determined that PhuZ forms dynamically unstable three-stranded filaments that are critical for centering replicated page in the host cell. These filaments use a unique C-terminal tail to polymerize and their minus ends are stabilized at the cell poles to orient growth towards the center to properly center phage. PhuZ is the first known prokaryotic tubulin to form a bipolar spindle and be dynamically unstable. The discovery of these properties has opened up a new frontier for the cell biology of viruses and provides new understanding of the evolution and GTP regulation of tubulin filaments.