Dynamics of the formin for3p in actin cable assembly.
BACKGROUND
Formins are a conserved family of actin nucleators responsible for the assembly of diverse actin structures such as cytokinetic rings and filopodia. In the fission yeast Schizosaccharomyces pombe, the formin for3p is necessary for the formation of actin cables, which are bundles of short parallel actin filaments that regulate cell polarity. These filaments are largely organized with their barbed ends facing the cell tip, where for3p is thought to function in their assembly.


RESULTS
Here, using a functional for3p-3GFP fusion expressed at endogenous levels, we find that for3p localizes to small dots that appear transiently at cell tips and then move away on actin cables at a rate of 0.3 microm/s. These movements were dependent on the continuous assembly of actin in cables, on the ability of for3p to bind actin within its FH2 domain, and on profilin and bud6p, two formin binding proteins that promote formin activity. Bud6p transiently colocalizes with for3p at the cell tip and stays behind at the cell tip when for3p detaches.


CONCLUSIONS
These findings suggest a new model for actin cable assembly: a for3p particle is activated and promotes the assembly of a short actin filament at the cell tip for only seconds. For3p and the actin filament may then be released from the cell tip and carried passively into the cell interior by retrograde flow of actin filaments in the cable. These studies reveal a complex and dynamic cycle of formin regulation and actin cable assembly in vivo.
Martin
Sophie G
SG
Department of Microbiology, College of Physicians and Surgeons, Columbia University, 701 West 168th Street, New York, New York 10032, USA.
Chang
Fred
F
D008841:Actin Cytoskeleton / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D036681:Fluorescence Recovery After Photobleaching; D000080865:Formins; D049452:Green Fluorescent Proteins / Q000032:analysis; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008954:Models, Biological; D051304:Profilins / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012568:Schizosaccharomyces / Q000166:cytology / Q000378:metabolism*; D029702:Schizosaccharomyces pombe Proteins / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D020741:rho GTP-Binding Proteins / Q000378:metabolism
D018797:Cell Cycle Proteins; C444419:FOR3 protein, S pombe; D000080865:Formins; D008869:Microtubule-Associated Proteins; D051304:Profilins; D029702:Schizosaccharomyces pombe Proteins; C501888:Tea4 protein, S pombe; C490133:cdc3 protein, S pombe; C489566:rho3 protein, S pombe; D049452:Green Fluorescent Proteins; D020741:rho GTP-Binding Proteins