Which of the following proteins nucleates actin polymerization and remains bound to the (+) end?
Spontaneous formation of actin nuclei is thermodynamically unfavorable because dimers and trimers dissociate rapidly, creating lag phase in polymerization assays. Formins overcome this by stabilizing nucleation intermediate and remaining processively attached. Family members such as mDia1, mDia2, INF2, FMNL and yeast Bni1 and Bnr1 contain FH1 proline rich region and FH2 donut shaped dimer. Each FH2 hemidimer contacts actin, encircling barbed end and forming stable dimer nucleus that templates addition of subsequent ATP actin subunits. FH1 recruits profilin ATP actin via polyproline interactions, increasing effective concentration near growing end up to fifty fold and accelerating elongation to over 100 subunits per second while protecting from capping proteins CapZ and gelsolin. Processive stepping mechanism allows formin to walk with barbed end as filament grows, generating long unbranched filaments that compose stress fibers, filopodia, lamellar arcs and cytokinetic contractile rings. This contrasts with Arp2/3 which branches, and with sequestering proteins thymosin beta4 and profilin alone regulating monomer pool.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Actin Nucleation by Formins and FH Domains.