A Processive Arabidopsis Formin Modulates Actin Filament Dynamics in Association with Profilin

Abstract

Formins are conserved regulators of actin cytoskeletal organization and dynamics that have been implicated to be important for cell division and cell polarity. The mechanism by which diverse formins regulate actin dynamics in plants is still not well understood. Using in vitro single-molecule imaging technology, we directly observed that the FH1-FH2 domain of an Arabidopsis thaliana formin, AtFH14, processively attaches to the barbed end of actin filaments as a dimer and slows their elongation rate by 90%. The attachment persistence of FH1-FH2 is concentration dependent. Furthermore, by use of the triple-color total internal reflection fluorescence microscopy, we found that ABP29, a barbed-end capping protein, competes with FH1-FH2 at the filament barbed end, where its binding is mutually exclusive with AtFH14. In the presence of different plant profilin isoforms, FH1-FH2 enhances filament elongation rates from about 10 to 42 times. Filaments buckle when FH1-FH2 is anchored specifically to cover slides, further indicating that AtFH14 moves processively on the elongating barbed end. At high concentration, AtFH14 bundles actin filaments randomly into antiparallel or parallel spindle-like structures; however, the FH1-FH2-mediated bundles become thinner and longer in the presence of plant profilins. This is the direct demonstration of a processive formin from plants. Our results also illuminate the molecular mechanism of AtFH14 in regulating actin dynamics via association with profilin.

LlFH1‐mediated interaction between actin fringe and exocytic vesicles is involved in pollen tube tip growth

“…It has been demonstrated that several plant formins, such as AtFH1, AtFH8, AtFH14, AtFH16 and OsFH5, bundle actin filaments (Michelot et al ., ; Li et al ., ; Xue et al ., ; Yang et al ., ; Zhang et al ., , ; Wang J et al ., ). To determine the binding activity of LlFH1 to actin filaments, we visualized actin filaments directly using fluorescence microscopy.…”

“…A number of studies have demonstrated that profilin increases the rate of actin filament elongation associated with formin FH1FH2 domains (Kovar et al, 2006;Vidali et al, 2009b;Zhang et al, 2016). We cloned three profilin isoforms from the mature pollen of Lilium, which have 76-88% identity.…”

“…Previous studies have shown that profilin sequesters the majority of actin monomers in plant cells and that the actin/profilin complex represents the physiologically relevant source of actin monomers in plants (Wang et al, 2005;Chaudhry et al, 2007). Recently, one class II formin AtFH14 from Arabidopsis has been demonstrated to accelerate actin elongation in the presence of plant profilin (Zhang et al, 2016). However, it is unclear whether pollen-specific class I formin interacts with profilin to promote the rate of actin elongation.…”

OsFH15, a class I formin, interacts with microfilaments and microtubules to regulate grain size via affecting cell expansion in rice

“…Several studies showed that profilin can increase rates of AF elongation associated with FH1FH2 domain of formins 18, 36, 37 . We selected OsPRF1 to investigate interaction effects of profilins and OsFH15 on regulating the elongation rate of AFs.…”

Rice Morphology Determinant-Mediated Actin Filament Organization Contributes to Pollen Tube Growth

“…Notably, both type I and type II formins have been demonstrated to accelerate actin elongation in the presence of plant profilins (Zhang et al, 2016b;Li et al, 2017). In Arabidopsis, profilins act as important players in regulating polarized pollen tube growth by modulating actin polymerization at the apical membrane .…”