Cross-links actin filaments (F-actin) in a calcium independent manner. Induces the formation of actin aggregates. Stabilizes and prevents F-actin depolymerization mediated by profilin. Key regulator of actin cytoarchitecture, probably involved in cell cycle, cell division, cell elongation and cytoplasmic tractus. Interacts with F-actin. A number of isoforms are produced. According to EST sequences. Constitutively expressed in roots, leaves, flowers and siliques, and to lower extent in stems. The actin-binding domain 1 (also called ABD1) seems to not bind F-actin alone while the second actin-binding domain (ABD2) can bind F-actin alone. EF-hand domain and the last 65 C-terminal amino acids are not required to cross-link F-actin, but enhance the stability of the binding. Cross-links actin with a constant of dissociation of 0.55 uM and stoichiometry of 4 molecules of F-actin for one molecule of FIM1.