We report that the actin assembly inhibitor latrunculin-A (LAT-A) causes complete disruption of the yeast actin cytoskeleton within 2–5 min, suggesting that although yeast are nonmotile, their actin filaments undergo rapid cycles of assembly and disassembly in vivo. Differences in the LAT-A sensitivities of strains carrying mutations in components of the actin cytoskeleton suggest that tropomyosin, fimbrin, capping protein, Sla2p, and Srv2p act to increase actin cytoskeleton stability, while End3p and Sla1p act to decrease stability. Identification of three LAT-A resistant actin mutants demonstrated that in vivo effects of LAT-A are due specifically to impairment of actin function and implicated a region on the three-dimensional actin structure as the LAT-A binding site.LAT-A was used to determine which of 19 different proteins implicated in cell polarity development require actin to achieve polarized localization. Results show that at least two molecular pathways, one actindependent and the other actin-independent, underlie polarity development. The actin-dependent pathway localizes secretory vesicles and a putative vesicle docking complex to sites of cell surface growth, providing an explanation for the dependence of polarized cell surface growth on actin function. Unexpectedly, several proteins that function with actin during cell polarity development, including an unconventional myosin (Myo2p), calmodulin, and an actin-interacting protein (Bud6/Aip3p), achieved polarized localization by an actin-independent pathway, revealing interdependence among cell polarity pathways. Finally, transient actin depolymerization caused many cells to abandon one bud site or mating projection and to initiate growth at a second site. Thus, actin filaments are also required for maintenance of an axis of cell polarity.

SUMMARY Foraging animals have distinct exploration and exploitation behaviors that are organized into discrete behavioral states. Here we characterize a neuromodulatory circuit that generates long-lasting roaming and dwelling states in Caenorhabditis elegans. We find that two opposing neuromodulators, serotonin and the neuropeptide pigment dispersing factor (PDF), each initiate and extend one behavioral state. Serotonin promotes dwelling states through the MOD-1 serotonin-gated chloride channel. The spontaneous activity of serotonergic neurons correlates with dwelling behavior, and optogenetic modulation of the critical MOD-1-expressing targets induces prolonged dwelling states. PDF promotes roaming states through a Gαs-coupled PDF receptor; optogenetic activation of cAMP production in PDF receptor-expressing cells induces prolonged roaming states. The neurons that produce and respond to each neuromodulator form a distributed circuit orthogonal to the classical wiring diagram, with several essential neurons that express each molecule. The slow temporal dynamics of this neuromodulatory circuit supplement fast motor circuits to organize long-lasting behavioral states.

Innate social behaviors emerge from neuronal circuits that interpret sensory information based on an individual's own genotype, sex, and experience. The regulated aggregation behavior of C. elegans, a simple animal with only 302 neurons, is an attractive system to analyze these circuits. Wild social strains of the nematode Caenorhabditis elegans aggregate in the presence of specific sensory cues, but solitary strains do not1,2,3,4. Here we identify the RMG inter/motor neuron as the hub of a regulated circuit that controls aggregation and related behaviors. RMG is the central site of action of the neuropeptide receptor gene npr-1, which distinguishes solitary strains (high npr-1 activity) from wild social strains (low npr-1 activity); high RMG activity is essential for all aspects of social behavior. Anatomical gap junctions connect RMG to multiple classes of sensory neurons known to promote aggregation, and to ASK sensory neurons, which are implicated in male attraction to hermaphrodite pheromones5. We find that ASK neurons respond directly to pheromones, and that high RMG activity enhances ASK responses in social strains, causing hermaphrodite attraction to pheromones at concentrations that repel solitary hermaphrodites. The coordination of social behaviors by RMG suggests an anatomical hub-and-spoke model for sensory integration in aggregation, and points to functions for related circuit motifs in the C. elegans wiring diagram.