The caspase-3 (CPP32, apopain, YAMA) family of cysteinyl proteases has been implicated as key mediators of apoptosis in mammalian cells. Gelsolin was identified as a substrate for caspase-3 by screening the translation products of small complementary DNA pools for sensitivity to cleavage by caspase-3. Gelsolin was cleaved in vivo in a caspase-dependent manner in cells stimulated by Fas. Caspase-cleaved gelsolin severed actin filaments in vitro in a Ca2+-independent manner. Expression of the gelsolin cleavage product in multiple cell types caused the cells to round up, detach from the plate, and undergo nuclear fragmentation. Neutrophils isolated from mice lacking gelsolin had delayed onset of both blebbing and DNA fragmentation, following apoptosis induction, compared with wild-type neutrophils. Thus, cleaved gelsolin may be one physiological effector of morphologic change during apoptosis.

Polymerized (F-)actin is induced to form bundles by a number of polycations including divalent metal ions, Co(NH 3 ) 6 3؉ , and basic polypeptides. The general features of bundle formation are largely independent of the specific structure of the bundling agent used. A threshold concentration of polycation is required to form lateral aggregates of actin filaments. The threshold concentration varies strongly with the valence of the cation and increases with the ionic strength of the solution. Polyanions such as nucleoside phosphates or oligomers of acidic amino acids disaggregate actin bundles into single filaments. These features are similar to the phenomenon of DNA condensation and can be explained analogously by polyelectrolyte theories. Similar results were found when F-actin was bundled by the peptide corresponding to the actin binding site of myristoylated alanine-rich protein kinase C substrate protein (MARCKS) or by smooth muscle calponin, suggesting that a broad class of actin bundling factors may function in a common manner. Physiologic concentrations of both small ions and large proteins can induce actin interfilament association independent of a requirement for specific binding sites.Actin polymerizes to double stranded filamentous form (Factin) in solutions of physiological ionic strength (2 mM MgCl 2 and 100 mM KCl). At relatively high (Ͼ10 mM) concentrations of divalent cations such as Mg 2ϩ , F-actin forms aggregates of various forms, characterized as types I, II, and III paracrystals (1). Stable Type III paracrystals appear as large and compact side-by-side aggregates, with additional morphological variations identified by analyzing electron micrographs (EM) (2-6). An EM specimen typically manifests several morphologies, for which no difference in experimental conditions can be assigned. The co-existence of these morphological states implies that the total free energies of the various bundle forms are similar, provided that an overall attractive interaction exists in order to bring the filaments together.Chemicals that cause F-actin to form paracrystalline bundles, such as divalent cations at high concentrations (order of 10 mM) (4), trivalent cations (mM range) (7), and polyamines (3, 4) are similar to those which cause DNA condensation (8, 9), except that the latter effect requires consistently higher concentrations of polycations. Both effects also occur at low pH (Ͻ5.5), and at high osmotic pressure (by addition of polyethylene glycol, for example) (10). It is the goal of this paper to demonstrate that the mechanism of F-actin bundle formation is analogous to that established for DNA condensation.The phenomenon of DNA condensation has been successfully treated by the theory of linear polyelectrolytes (11)(12)(13)(14). A double stranded DNA at neutral pH has a linear charge spacing b ϭ 1.7 Å, much less than the Bjerrum length, the distance between elementary charges at which the electrostatic interaction energy equals the thermal energy kT, i.e. B ϭ e 2 /4⑀ 0 ⑀ kT. In water, for example, the di...

In this letter we propose a kinematic model to explain how collisions with a surface and rotational Brownian motion give rise to accumulation of micro-swimmers near a surface. In this model, an elongated microswimmer invariably travels parallel to the surface after hitting it from an oblique angle. It then swims away from the surface, facilitated by rotational Brownian motion. Simulations based on this model reproduce the density distributions measured for the small bacteria E. coli and Caulobacter crescentus, as well as for the much larger bull spermatozoa swimming between two walls.Swimming aids the function of microorganisms, such as enhancing the formation of biofilms on surfaces [1]. Swimming also helps transport sperms toward eggs for fertilization [2]. The density of cells as a function of distance from a surface has been measured for swimming E. coli [3] and bull spermatozoa [4], showing interestingly in both cases values much higher near the surface than far away. This near surface accumulation has mainly been attributed to the hydrodynamic attraction between the cells and the surface [4,5]. Recently, Berke et al. [3] combined the effects of the hydrodynamic attraction and the translational Brownian motion of the cells to predict the distribution of E. coli as a function of distance. As noted by the authors [3], however, this interpretation is not applicable to cells within 10 μm from the surface, where the cell density is the highest. The hydrodynamic interaction among the microswimmers has been shown to be important only at high cell concentrations [6,7].In this letter we present a different account for the near surface accumulation. We ignore the hydrodynamic attraction but emphasize the role of collision with a surface at a low Reynolds number [8], an interaction that deflects the swimming direction, and the role of rotational Brownian motion of individual microswimmers in a confined environment. We show that a typical microswimmer with an elongated shape tends to swim parallel to a surface after hitting it at an oblique angle and therefore accumulate near the surface. Rotational Brownian motion [9] then relaxes the accumulation by randomly changing the swimming direction so that the cells have chances to swim away from the surface. In the extreme case of no rotational Brownian motion, all the cells would end up swimming in close proximity with the surface. In the opposite extreme of very fast rotational Brownian motion, the cells will quickly change to any possible swimming direction and subsequently would be found anywhere with equal probability. In reality, a microswimmer randomly changes its swimming direction with a finite rotational diffusion constant, resulting in a distribution in between the two extremes, that is, more cells stay near the surface and fewer far away.We used the bacterium C. crescentus strain CB15 SB3860 to examine the near surface swimming and accumulation. Swarmer cells of this mutant swim forward only and do not *Jay_Tang@brown.edu. . We noted that although this ...