The Ultrastructure and Nucleotide-regulated Assembly Mechanism of the Bacteriophage-encoded Tubulin Homologue, PhuZ

Tubulin is a universally conserved molecule, found in all three domains of life. Tubulin filaments use energy derived from GTP binding and hydrolysis to organize cytoplasm. Although tubulin homologues share the same fold to their monomers and similar protofilament architecture, they assemble filaments with unique geometric designs. Moreover, despite sharing the mechanism of self-assembly, wherein tubulin subunits require GTP binding to polymerize, and their polymerization stimulates GTP hydrolysis, tubulins exhibit distinctive dynamic properties. Together, unique filament architectures and dynamic properties determine a specific set of biological functions each tubulin family performs. The origins of tubulin filament architectural diversity and distinctive dynamic behavior are not well understood. We have been studying PhuZ tubulins, which are encoded by a few very large Pseudomonas ϕKZ-like bacteriophages. Our studies have shown that PhuZ assembles dynamically unstable spindle-like arrays that organize bacteriophage DNA at the cell midpoint, which somehow facilitates phage infectivity. Moreover, PhuZ monomer structure has the canonical tubulin fold, with a unique, highly conserved and extended C-terminus. The C-terminus mediates protofilament contacts and is critical for polymerization both in vitro and in vivo. The main focus of this manuscript is the structural investigation of the molecular mechanisms underlying PhuZ dynamic behavior. In an attempt to explain how GTP binding and hydrolysis drives PhuZ filament turnover, we have conducted a number of electron cryomicroscopy studies (cryo-EM) on PhuZ filaments in both pre- and post- hydrolysis states. PhuZ forms a polar three-stranded polymer with an unusual subunit orientation. Its C-terminus guides a cooperative assembly of the three-stranded filament by mediating both longitudinal and lateral interactions. Upon assembly, the longitudinal interface, C-terminus and tubulin fold undergo polymerization-competent rearrangements. We propose that the energy of GTP binding is stored in the displacement of these structural elements. Also, our structural studies have revealed how the energy of GTP hydrolysis is stored in the PhuZ filament lattice. In particular, GTP hydrolysis, initially sensed by the T3 loop and C-terminus, is accompanied by unwinding and supercoiling of the filament lattice. Based on these observations, we propose an assembly/disassembly pathway of PhuZ filament

The Structure and Assembly Mechanism of a Novel Three-Stranded Tubulin Filament that Centers Phage DNA

Tubulins are a universally conserved protein superfamily that carry out diverse biological roles by assembling filaments with very different architectures. The underlying basis of this structural diversity is poorly understood. Here, we determine a 7.1 Å cryo-electron microscopy reconstruction of the bacteriophage-encoded PhuZ filament and provide molecular-level insight into its cooperative assembly mechanism. The PhuZ family of tubulins is required to actively center the phage within infected host cells, facilitating efficient phage replication. Our reconstruction and derived model reveal the first example of a three-stranded tubulin filament. We show that the elongated C-terminal tail simultaneously stabilizes both longitudinal and lateral interactions, which in turn define filament architecture. Identified interaction surfaces are conserved within the PhuZ family, and their mutagenesis compromises polymerization in vitro and in vivo. Combining kinetic modeling of PhuZ filament assembly and structural data, we suggest a common filament structure and assembly mechanism for the PhuZ family of tubulins

Three-dimensional ultrastructure of the septin filament network in Saccharomyces cerevisiae.

Septins are conserved GTP-binding proteins involved in membrane compartmentalization and remodeling. In budding yeast, five mitotic septins localize at the bud neck, where the plasma membrane is enriched in phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P(2)). We previously established the subunit organization within purified yeast septin complexes and how these hetero-octamers polymerize into filaments in solution and on PtdIns4,5P(2)-containing lipid monolayers. How septin ultrastructure in vitro relates to the septin-containing filaments observed at the neck in fixed cells by thin-section electron microscopy was unclear. A morphological description of these filaments in the crowded space of the cell is challenging, given their small cross section. To examine septin organization in situ, sections of dividing yeast cells were analyzed by electron tomography of freeze-substituted cells, as well as by cryo-electron tomography. We found networks of filaments both perpendicular and parallel to the mother-bud axis that resemble septin arrays on lipid monolayers, displaying a repeat pattern that mirrors the molecular dimensions of the corresponding septin preparations in vitro. Thus these in situ structures most likely represent septin filaments. In viable mutants lacking a single septin, in situ filaments are still present, although more disordered, consistent with other evidence that the in vivo function of septins requires filament formation

A Phage Tubulin Assembles Dynamic Filaments by an Atypical Mechanism to Center Viral DNA within the Host Cell

Tubulins are essential for the reproduction of many eukaryotic viruses, but historically, bacteriophage were assumed not to require a cytoskeleton. Here, we identify a tubulin-like protein, PhuZ, from bacteriophage 201φ2-1 and show that it forms filaments in vivo and in vitro. The PhuZ structure has a conserved tubulin fold, with an unusual, extended C terminus that we demonstrate to be critical for polymerization in vitro and in vivo. Longitudinal packing in the crystal lattice mimics packing observed by EM of in-vitro-formed filaments, indicating how interactions between the C terminus and the following monomer drive polymerization. PhuZ forms a filamentous array that is required for positioning phage DNA within the bacterial cell. Correct positioning to the cell center and optimal phage reproduction only occur when the PhuZ filament is dynamic. Thus, we show that PhuZ assembles a spindle-like array that functions analogously to the microtubule-based spindles of eukaryotes

Three-dimensional ultrastructure of the septin filament network in Saccharomyces cerevisiae

Septins are essential for membrane compartmentalization and remodeling. Electron tomography of yeast bud necks shows filaments perpendicular and parallel to the mother-bud axis that resemble in vitro septin arrays. Filaments are still present, although disordered, in mutants lacking a single septin, underscoring the importance of septin assembly