The Ras-like small G-protein MglA is an integral part of the gliding motility complex at bacterial focal adhesions and stimulates the assembly of the motility complex by directly connecting it to the MreB actin cytoskeleton.

The assembly of static supramolecular structures is a culminating event of developmental programs. One such structure, the proteinaceous shell (called the coat) that surrounds spores of the bacterium Bacillus subtilis, is composed of about 70 different proteins and represents one of the most durable biological structures known. The coat is built atop a basement layer that contains an ATPase (SpoIVA) that forms a platform required for coat assembly. Here, we show that SpoIVA belongs to the translation factors class of P-loop GTPases and has evolutionarily lost the ability to bind GTP; instead, it uses ATP hydrolysis to drive its self-assembly into static filaments. We demonstrate that ATP hydrolysis is required by every subunit for incorporation into the growing polymer by inducing a conformational change that drives polymerization of a nucleotide-free filament. SpoIVA therefore differs from other self-organizing polymers (dynamic cytoskeletal structures and static intermediate filaments) in that it uses ATP hydrolysis to self-assemble, not disassemble, into a static polymer. We further show that polymerization requires a critical concentration that we propose is only achieved once SpoIVA is recruited to the surface of the developing spore, thereby ensuring that SpoIVA polymerization only occurs at the correct subcellular location during spore morphogenesis.he assembly of static structures represents an important developmental end point that can contribute to the characteristic morphology of an organism. Unlike dynamic structures, such as those made of actin and tubulin, that are frequently assembled and disassembled to suit the needs of a cell at a particular time, large static structures, such as eggshells, flagella, and teeth, are built to remain intact for longer periods of time (1-4). Among the most durable structures in biology is the proteinaceous shell, called the coat, that surrounds the dormant endospores of Grampositive bacteria and self-assembles to create a structure that may last for many years (5-9). Indeed, this structure contributes to the remarkable resilience of major pathogens like Bacillus anthracis and Clostridium difficile against environmental onslaughts (10, 11).Spore formation (sporulation) in the rod-shaped bacterium Bacillus subtilis initiates when the cell senses the imminent deprivation of nutrients (12-15). The bacterium responds to this starvation condition by dividing asymmetrically and elaborating a spherical internal organelle, called the forespore, that is enveloped by a double membrane and contains a copy of the genetic material (Fig. 1A). The outer cell (the "mother cell") nurtures the forespore as it matures into a largely dormant cell; at that point, the mother cell lyses and releases the now mature spore into the environment. Part of this nurturing consists of the deposition of some 70 different proteins produced in the mother cell onto the surface of the forespore in a highly coordinated manner that eventually will form the coat, the outermost feature of mature B. subtili...

In bacteria, certain shape-sensing proteins localize to differently curved membranes. During sporulation in Bacillus subtilis, the only convex (positively curved) surface in the cell is the forespore, an approximately spherical internal organelle. Previously, we demonstrated that SpoVM localizes to the forespore by preferentially adsorbing onto slightly convex membranes. Here, we used NMR and molecular dynamics simulations of SpoVM and a localization mutant (SpoVM P9A ) to reveal that SpoVM's atypical amphipathic α-helix inserts deeply into the membrane and interacts extensively with acyl chains to sense packing differences in differently curved membranes. Based on binding to spherical supported lipid bilayers and Monte Carlo simulations, we hypothesize that SpoVM's membrane insertion, along with potential cooperative interactions with other SpoVM molecules in the lipid bilayer, drives its preferential localization onto slightly convex membranes. Such a mechanism, which is distinct from that used by high curvature-sensing proteins, may be widely conserved for the localization of proteins onto the surface of cellular organelles.ArfGAP1 | SpoIVA | DivIVA | curvature sensing | lipid packing