Fimbrins

Fimbrins are a family of actin-binding proteins that crosslink actin filaments into tight, parallel bundles, contributing to the organization and mechanical properties of the actin cytoskeleton in many eukaryotic cells. By forming bundled networks rather than loose gels, fimbrins help shape microvilli, stereocilia, and other bundled actin structures, and they participate in processes such as cell adhesion and migration.

Structural features: A typical fimbrin molecule contains two actin-binding domains (ABD1 and ABD2), each consisting of two calponin-homology (CH) domains, arranged to bind two filaments simultaneously and promote crosslinking into bundles. Many fimbrins also have regulatory regions, and some family members possess calcium-binding motifs or other regulatory modules that respond to cellular signals to modulate bundling activity.

Diversity and localization: In vertebrates, fimbrins are commonly referred to as plastins and include several tissue-specific isoforms that localize to microvilli in epithelia, stereocilia in sensory cells, or the cortical actin network in other cells. In yeast and other fungi, Sac6 is a representative fimbrin involved in actin bundle formation.

Regulation and interactions: Bundling by fimbrins is influenced by phosphorylation, calcium signaling, and interaction with other actin-binding proteins. Fimbrins can cooperate with or compete with other crosslinkers, such as fascin or alpha-actinin, to shape the architecture of actin networks.

Clinical and functional significance: Proper fimbrin function is important for maintaining the integrity of microvilli and sensory structures and for normal immune cell function in some organisms. Altered expression or function can affect cell morphology and motility.