The Structurally Plastic CH2 Domain Is Linked to Distinct Functions of Fimbrins/Plastins

Abstract

Fimbrins/plastins have been implicated in the generation of distinct actin structures, which are linked to different cellular processes. Historically, fimbrins/plastins were mainly considered as generating tight actin bundles. Here, we demonstrate that different members of the fimbrin/plastin family have diverged biochemically during evolution to generate either tight actin bundles or loose networks with distinct biochemical and biophysical properties. Using the phylogenetically and functionally distinct Arabidopsis fimbrins FIM4 and FIM5 we found that FIM4 generates both actin bundles and cross-linked actin filaments, whereas FIM5 only generates actin bundles. The distinct functions of FIM4 and FIM5 are clearly observed at singlefilament resolution. Domain swapping experiments showed that cooperation between the conformationally plastic calponinhomology domain 2 (CH2) and the N-terminal headpiece determines the function of the full-length protein. Our study suggests that the structural plasticity of fimbrins/plastins has biologically meaningful consequences, and provides novel insights into the structure-function relationship of fimbrins/plastins as well as shedding light on how cells generate distinct actin structures.

Organizational Innovation of Apical Actin Filaments Drives Rapid Pollen Tube Growth and Turning

“…We therefore investigated tip-directed vesicle transport in Arabidopsis strains with altered filament binding status due to mutations in genes encoding actin-binding proteins. Indeed, we found that fim5 mutant pollen tubes, which have disorganized apical actin filaments (Supplemental Figure 4A; Zhang et al, 2016aZhang et al, , 2016b, exhibited defects in apical vesicle accumulation (Supplemental Figure 4B and 4C). To determine whether cortical or internal or both types of apical actin filament are vital for tip-directed vesicle transport, we performed FRAP in pollen tubes expressing Lat52::YFP-RabA4b.…”

“…To visualize the arrangement and dynamics of actin filaments in tobacco and lily pollen tubes, microparticle bombardment was employed according to previously published papers (Twell et al, 1989;Zhang et al, 2016b). The expression of Lifeact-eGFP in tobacco and lily pollen tubes was under the control of the promoters of Lat52 and Zm13, respectively (Hamilton et al, 1992).…”

Targeted deletion of the zebrafish actin-bundling protein L-plastin (lcp1)

“…Each L-plastin monomer can bind two adjacent molecules of filamentous actin, stabilizing the parallel strands. Illustration by ZJC; L-plastin structure based on [ 8 ].…”

Arabidopsis FIM4 and FIM5 regulates the growth of root hairs in an auxin-insensitive way

“…It is necessary for pollen germination and pollen tube growth. [36][37][38] Except AtFIM5, chip data showed that AtFIM4 was also expressed in pollen tubes. Biochemistry data showed AtFIM4 made microfilaments bundle and branch, while AtFIM5 could only make microfilament bundle.…”

“…Biochemistry data showed AtFIM4 made microfilaments bundle and branch, while AtFIM5 could only make microfilament bundle. 38 In vivo, AtFIM4 acts coordinately with AtFIM5 to organize and maintain the longitudinal actin bundles in pollen tubes. 39 In this study, the wild type (WT) and mutant seeds (atfim4-1/atfim5-2) were spread on 1/2MS medium with 250 nM indole-3-acetic acid (IAA), and after seven days germination, the root hair length was measured.…”