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 interaction
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Actin Nucleation by Formins and FH Domains.