Coherent structures appear in a concentrated suspension of swimming bacteria. While transport phenomena in a suspension have been studied extensively, how energy is transported from the individual cell scale to the larger mesoscale remains unclear. In this study, we carry out the first successful measurement of the three-dimensional velocity field in a dense suspension of bacteria. The results show that most of the energy generated by individual bacteria dissipates on the cellular scale. Only a small amount of energy is transported to the mesoscale, but the gain in swimming velocity and mass transport due to mesoscale coherent structures is enormous. These results indicate that collective swimming of bacteria is efficient in terms of energy. This paper sheds light on how energy can be transported toward smaller wave numbers in the Stokes flow regime.

We have developed a novel system for expressing recombinant actin in Dictyostelium. In this system, the C terminus of actin is fused to thymosin ␤ via a glycine-based linker. The fusion protein is purified using a His tag attached to the thymosin ␤ moiety and then cleaved by chymotrypsin immediately after the native final residue of actin to yield intact actin. Wildtype actin prepared in this way was functionally normal in terms of its polymerization kinetics and muscle myosin-mediated motility. We expected that this system would be particularly useful for expressing toxic actin mutants, because the actin moiety of the fusion protein is unlikely to interact with the actin cytoskeleton of the host cells. We therefore chose to express the E206A/R207A/E208A mutant, which appears to be dominant lethal in yeast, as a model case of a toxic actin mutant that is difficult to express. We found that the E206A/R207A/E208A mutant could be expressed and purified with a yield comparable to the wild-type molecule (3-4 mg/20 g cells), even though green fluorescent protein-fused actin carrying the E206A/ R207A/E208A mutation was expressed at a much lower level than wild-type actin. Purified E206A/R207A/E208A actin did not polymerize, even in the presence of muscle actin; however, it accelerated polymerization of muscle actin and inhibited the nucleating and severing activities of gelsolin. Given that the location of the substituted residues is near the pointed end face of the mutant, we suggest that E206A/R207A/E208A actin behaves like a weak pointed end-capping protein that perturbs the actin cytoskeleton of the host cells.Actin filaments in live cells undergo continuous, dynamic turnover and remodeling. These processes involve polymerization, depolymerization, severing, capping, and branching of actin filaments through interaction with a vast array of actinbinding proteins. At present, understanding how the activities of these proteins are regulated is a major issue in cell biology. In addition, it is known that subunits within actin filaments are able to take different conformations and that, in certain cases, large blocks of subunits spanning long distances within individual filaments undergo cooperative conformational changes (1-6). For instance, in the case of thin filaments in striated muscle, Ca 2ϩ -triggered conformational changes in the filaments are enhanced by cooperative conformational changes induced by myosin (reviewed in Ref. 7). In other cases, however, the physiological function of cooperative conformational changes in actin subunits remains unclear. Thus, a number of interesting questions as to the structure and function of actin filaments remain unanswered. Dominant negative actin mutations have been identified from genetic screens, and these mutants may be useful for the elucidation of actin functions, because the use of dominant negative mutant proteins often provides a unique means of dissecting the molecular mechanisms underlying complex phenomena, particularly those that involve direct interaction among ...

Dynein is a motor protein that hydrolyses ATP and moves toward the minus end of a microtubule (MT). A dynein molecule has one to three heavy chains, each consisting of three domains: a head, a stalk and a tail. ATP is bound and hydrolysed in the head, which has a ring-like structure composed of 6 AAA+ domains. The stalk is an antiparallel coiled-coil, 10–15 nm long, and has a nucleotide-dependent MT-binding domain at the tip (1) (Fig. 1). It has been proposed that the nucleotide-dependent binding affinity of the tubulin-binding site at the tip of the stalk is modulated by the two alpha helices in the coiled-coil sliding over each other (2). However, it is not known how a dynein molecule moves along a microtubule (MT).

Eukaryotic translation elongation factor 1A (eEF1A) is known to be a multifunctional protein. In Tetrahymena, eEF1A is localized to the division furrow and has the character to bundle filamentous actin (F-actin). eEF1A binds F-actin and the ratio of eEF1A and actin is approximately 1:1 (Kurasawa et al., 1996). In this study, we revealed that eEF1A itself exists as monomer and dimer, using gel filtration column chromatography. Next, eEF1A monomer and eEF1A dimer were separated using gel filtration column, and their interaction with F-actin was examined with cosedimentation assay and electron microscopy. In the absence of Ca2+/calmodulin (CaM), eEF1A dimer bundled F-actin and coprecipitated with F-actin at low-speed centrifugation, but eEF1A monomer did not. In the presence of Ca2+/CaM, eEF1A monomer increased, while dimer decreased. To examine that Ca2+/CaM alters eEF1A dimer into monomer and inhibits bundle formation of F-actin, Ca2+/CaM was added to F-actin bundles formed by eEF1A dimer. Ca2+/CaM separated eEF1A dimer to monomer, loosened F-actin bundles and then dispersed actin filaments. Simultaneously, Ca2+/CaM/ eEF1A monomer complexes were dissociated from actin filaments. Therefore, Ca2+/CaM reversibly regulates the F-actin bundling activity of eEF1A.