Formin proteins nucleate actin filaments, remaining processively associated with the fast-growing barbed ends. Although formins possess common features, the diversity of functions and biochemical activities raised the possibility that formins differ in fundamental ways. Further, a recent study suggested that profilin and ATP hydrolysis are both required for processive elongation mediated by the formin mDia1. We used total internal reflection fluorescence microscopy to observe directly individual actin filament polymerization in the presence of two mammalian formins (mDia1 and mDia2) and two yeast formins (Bni1p and Cdc12p). We show that these diverse formins have the same basic properties: movement is processive in the absence or presence of profilin; profilin accelerates elongation; and actin ATP hydrolysis is not required for processivity. These results suggest that diverse formins are mechanistically similar, but the rates of particular assembly steps vary.

A new scanning probe-based microrheology approach is used to quantify the frequency-dependent viscoelastic behavior of both fibroblast cells and polymer gels. The scanning probe shape was modified using polystyrene beads for a defined surface area nondestructively deforming the sample. An extended Hertz model is introduced to measure the frequency-dependent storage and loss moduli even for thin cell samples. Control measurements of the polyacrylamide gels compare well with conventional rheological data. The cells show a viscoelastic signature similar to in vitro actin gels.

Viscoelasticity of the leading edge, i.e., the lamellipodium, of a cell is the key property for a deeper understanding of the active extension of a cell's leading edge. The fact that the lamellipodium of a cell is very thin (<1000 nm) imparts special challenges for accurate measurements of its viscoelastic behavior. It requires addressing strong substrate effects and comparatively high stresses (>1 kPa) on thin samples. We present the method for an atomic force microscopy-based microrheology that allows us to fully quantify the viscoelastic constants (elastic storage modulus, viscous loss modulus, and the Poisson ratio) of thin areas of a cell (<1000 nm) as well as those of thick areas. We account for substrate effects by applying two different models-a model for well-adhered regions (Chen model) and a model for nonadhered regions (Tu model). This method also provides detailed information about the adhered regions of a cell. The very thin regions relatively near the edge of NIH 3T3 fibroblasts can be identified by the Chen model as strongly adherent with an elastic strength of approximately 1.6 +/- 0.2 kPa and with an experimentally determined Poisson ratio of approximately 0.4 to 0.5. Further from the edge of these cells, the adherence decreases, and the Tu model is effective in evaluating its elastic strength ( approximately 0.6 +/- 0.1 kPa). Thus, our AFM-based microrheology allows us to correlate two key parameters of cell motility by relating elastic strength and the Poisson ratio to the adhesive state of a cell. This frequency-dependent measurement allows for the decomposition of the elastic modulus into loss and storage modulus. Applying this decomposition and Tu's and Chen's finite depth models allow us to obtain viscoelastic signatures in a frequency range from 50 to 300 Hz, showing a rubber plateau-like behavior.

Certain types of intracellular organelle transport to the cell periphery are thought to involve long-range movement on microtubules by kinesin with subsequent handoff to vertebrate myosin Va (myoVa) for local delivery on actin tracks. This process may involve direct interactions between these two processive motors. Here we demonstrate using single molecule in vitro techniques that myoVa is flexible enough to effectively maneuver its way through actin filament intersections and Arp2/3 branches. In addition, myoVa surprisingly undergoes a one-dimensional diffusive search along microtubules, which may allow it to scan efficiently for kinesin and/or its cargo. These features of myoVa may help ensure efficient cargo delivery from the cell center to the periphery.cytoskeleton ͉ molecular motor ͉ motility ͉ processivity C oordinated organelle transport along certain secretory pathways starts with kinesin-powered movement on microtubules and finishes with vertebrate myosin Va (myoVa)-based motility on actin (1-4). This trip requires that both motors be present when a microtubule-actin intersection is encountered, but how these two motors find one another or the cargo they share is still unknown. Another challenge is to understand how cargo is delivered to its destination by way of the complex cytoskeletal network. Vertebrate myoVa is a processive motor that can travel long distances along its actin track (5). However, the cytoskeletal intersections that myoVa must encounter along its journey may present a physical barrier to forward motion or an alternate path to its final destination.To investigate this issue from myoVa's point of view, we observed single myoVa molecules labeled with highly photostable quantum dots (Qdots) in an objective-type total internal reflectance microscope (6) as they maneuvered through a model of cytoskeletal intersections created on a microscope coverslip coated with either actin filaments (Fig. 1A) or actin filaments and microtubules (Fig. 1B). In this model system, actin-actin intersections could inhibit or alter myoVa's direction of travel while actin-microtubule intersections should act as a hurdle to myoVa's processive movement. Given myoVa's ability to take 72-nm steps as it walks processively along actin filaments in a handover-hand fashion (6-9) and its inherent flexibility (10, 11), we have observed that myoVa can easily maneuver past actin filament intersections. In contrast, when encountering a microtubule, myoVa cannot step over this physical barrier, but surprisingly can step onto the microtubule and begin a onedimensional diffusive search. Thus, myoVa has evolved to handle the challenges of the cytoskeletal network and through its interaction with the microtubule can effectively scan for its transport partner, kinesin, and/or its cargo. Results and Discussion MyoVa Dynamics at Actin Filament Intersections.To determine what happens when myoVa encounters an overlapping actin filament, we adhered Alexa Fluor 660 phalloidin-labeled actin filaments to the coverslip, followed by TRITC...

The cellular cytoskeleton is a complex dynamical network that constantly remodels as cells divide and move. This reorganization process occurs not only at the cell membrane, but also in the cell interior (bulk). During locomotion, regulated actin assembly near the plasma membrane produces lamellipodia and filopodia. Therefore, most in vitro experiments explore phenomena taking place in the vicinity of a surface. To understand how the molecular machinery of a cell self-organizes in a more general way, we studied bulk polymerization of actin in the presence of actin-related protein 2͞3 complex and a nucleation promoting factor as a model for actin assembly in the cell interior separate from membranes. Bulk polymerization of actin in the presence of the verprolin homology, cofilin homology, and acidic region, domain of WiskottAldrich syndrome protein, and actin-related protein 2͞3 complex results in spontaneous formation of diffuse aster-like structures. In the presence of fascin these asters transition into stars with bundles of actin filaments growing from the surface, similar to star-like structures recently observed in vivo. The transition from asters to stars depends on the ratio [fascin]͞[G actin]. The polarity of the actin filaments during the transition is preserved, as in the transition from lamellipodia to filopodia. Capping protein inhibits star formation. Based on these experiments and kinetic Monte Carlo simulations, we propose a model for the spontaneous selfassembly of asters and their transition into stars. This mechanism may apply to the transition from lamellipodia to filopodia in vivo.actin self-assembly ͉ asters ͉ cellular protrusions ͉ Monte Carlo simulations ͉ stars D uring cellular migration, regulated actin assembly takes place at the plasma membrane (1-3), with continuous disassembly deeper in the cell interior. The site-directed actin polymerization at the plasma membrane results in the extension of cellular protrusions in the form of lamellipodia and filopodia. Although most cultured animal cells assemble both lamellipodia and filopodia, some cells, like keratocytes, emphasize lamellipodia, whereas dendritic cells are dominated by filopodia. One open question is how does a cell ''choose'' between the formation of lamellipodia and filopodia and what dictates the preference of one structure over the other. For that, it is essential to understand the self-assembly of these actin-based structures and the factors controlling these processes.Although protrusions of lamellipodia and filopodia are tightly coupled to actin polymerization, the distinct organization and generation of filaments in each structure uses a different mechanism to produce mechanical force. In the lamellipodia, the actin filaments organize into a flat 2D branched network (2, 4), whereas bundles of parallel actin filaments support filopodia (5). In the lamellipodia, the branched nucleation is driven by activation of the actin-related protein (Arp) 2͞3 complex (6) by the Wiskott-Aldrich syndrome protein family (7-8), followed...