4 research outputs found
The functionally distinct fission yeast formins have specific actin-assembly properties
The three fission yeast formins (Cdc12, For3, and Fus1) all nucleate actin assembly and remain continuously associated with the elongating actin filament barbed end, while incorporating thousands of actin monomers before dissociating. However, the specific rates for these reactions vary significantly and may therefore be functionally important
The Cytokinesis Formins from the Nematode Worm and Fission Yeast Differentially Mediate Actin Filament Assembly*
Formins drive actin filament assembly for diverse cellular processes
including motility, establishing polarity, and cell division. To investigate
the mechanism of contractile ring assembly in animal cells, we directly
compared the actin assembly properties of formins required for cytokinesis in
the nematode worm early embryo (CYK-1) and fission yeast (Cdc12p). Like Cdc12p
and most other formins, CYK-1 nucleates actin filament assembly and remains
processively associated with the elongating barbed end while facilitating the
addition of profilin-actin above the theoretical diffusion-limited rate.
However, specific properties differ significantly between Cdc12p and CYK-1.
Cdc12p efficiently nucleates filaments that in the presence of profilin
elongate at approximately the same rate as control filaments without formin
(ā¼10.0 subunits/s). CYK-1 is an inefficient nucleator but allows filaments
to elongate profilin-actin 6-fold faster than Cdc12p (ā¼60 subunits/s).
Both Cdc12p and CYK-1 bind to pre-assembled actin filaments with low nanomolar
affinity, but CYK-1 dissociates 2 orders of magnitude more quickly. However,
CYK-1 rapidly re-associates with free barbed ends. Cdc12p allows barbed ends
to elongate in the presence of excess capping protein, whereas capping protein
inhibits CYK-1-mediated actin assembly. Therefore, these evolutionarily
diverse formins can drive contractile ring assembly by a generally similar
mechanism, but cells with unique dimensions and physical parameters might
require proteins with carefully tuned actin assembly properties
Role of Tropomyosin in Formin-mediated Contractile Ring Assembly in Fission Yeast
Like animal cells, fission yeast divides by assembling actin filaments into a contractile ring. In addition to formin Cdc12p and profilin, the single tropomyosin isoform SpTm is required for contractile ring assembly. Cdc12p nucleates actin filaments and remains processively associated with the elongating barbed end while driving the addition of profilin-actin. SpTm is thought to stabilize mature filaments, but it is not known how SpTm localizes to the contractile ring and whether SpTm plays a direct role in Cdc12p-mediated actin polymerization. Using ābulkā and single actin filament assays, we discovered that Cdc12p can recruit SpTm to actin filaments and that SpTm has diverse effects on Cdc12p-mediated actin assembly. On its own, SpTm inhibits actin filament elongation and depolymerization. However, Cdc12p completely overcomes the combined inhibition of actin nucleation and barbed end elongation by profilin and SpTm. Furthermore, SpTm increases the length of Cdc12p-nucleated actin filaments by enhancing the elongation rate twofold and by allowing them to anneal end to end. In contrast, SpTm ultimately turns off Cdc12p-mediated elongation by ātrappingā Cdc12p within annealed filaments or by dissociating Cdc12p from the barbed end. Therefore, SpTm makes multiple contributions to contractile ring assembly during and after actin polymerization