Formins are highly conserved proteins that have essential roles in remodelling the actin and microtubule cytoskeletons to influence eukaryotic cell shape and behaviour. Recent work has identified numerous cellular factors that locally recruit, activate or inactivate formins to bridle and unleash their potent effects on actin nucleation and elongation. The effects of formins on microtubules have also begun to be described, which places formins in a prime position to coordinate actin and microtubule dynamics. The emerging complexity in the mechanisms governing formins mirrors the wide range of essential functions that they perform in cell motility, cell division and cell and tissue morphogenesis.

Cells require actin nucleators to catalyze the de novo assembly of filaments and actin elongation factors to control the rate and extent of polymerization. Nucleation and elongation factors identified to date include Arp2/3 complex, formins, Ena/VASP, and newcomers Spire, Cobl, and Lmod. Here, we discuss recent advances in understanding their activities and mechanisms, and new evidence for their cooperation and interaction in vivo. Earlier models had suggested that different nucleators function independently to assemble distinct actin arrays. However, more recent observations indicate that the construction of most cellular actin networks depends on the activities of multiple actinassembly promoting factors working in concert.

SUMMARY Normal cellular development and function requires tight spatiotemporal control of actin assembly. Formins are potent actin assembly factors that protect the growing ends of filaments from capping proteins. However, it is unresolved how the duration of formin-mediated actin assembly events is controlled, whether formins are actively displaced from growing ends, and how filament length is regulated in vivo. Here, we identify Bud14 as a high affinity inhibitor of the yeast formin Bnr1 that rapidly displaces the Bnr1 FH2 domain from growing barbed ends. Consistent with these activities, bud14Δ cells display fewer actin cables, which are aberrantly long, bent, and latrunculinA-resistant, leading to defects in secretory vesicle movement. Moreover, bud14Δ suppressed mutations that cause abnormally numerous and shortened cables, restoring wild type actin architecture. From these results, we propose that formin displacement factors regulate filament length and are required in vivo to maintain proper actin network architecture and function.

may act indirectly to promote GFAP expression. We provide evidence that this indirect effect may be mediated via induction of immediate early genes and the transcription factor Sp1 by demonstrating that these transcriptional regulators are induced by BMP2 and contribute to GFAP promoter activity. These findings demonstrate new roles for intracellular kinase pathways in mediating the effects of BMPs during the early stages of glial differentiation and suggest that differential contributions by signaling and transcriptional networks may contribute to the range of effects of BMPs on neuronal and glial development during the formation of the peripheral nervous system.