fimbrin

Fimbrin, also known as plastin in some contexts, is a family of actin-bundling proteins that organize actin filaments into tight, parallel bundles. This bundling activity supports the formation and maintenance of actin-rich structures such as microvilli and stereocilia, and it contributes to the broader organization of the cytoskeleton in various cell types.

Structurally, fimbrin/plastin proteins possess two actin-binding domains, each comprising two calponin-homology (CH) domains, arranged to cross-link filaments into bundles. Many members of the family include regulatory EF-hand motifs in their connecting regions, enabling calcium-dependent modulation of their bundling activity. This regulatory mechanism allows dynamic remodeling of actin structures in response to Ca2+ signaling.

In vertebrates, several plastin/fimbrin isoforms are expressed in a tissue-specific manner, yet all share the core ability to bind and organize actin filaments. Fimbrin is especially associated with specialized actin structures, including intestinal microvilli and hair-cell stereocilia, and also participates in cytoplasmic actin networks that influence cell shape, adhesion, and motility.

Regulation of fimbrin’s activity occurs through intracellular calcium levels and post-translational modifications, which can alter its bundling strength and dynamics. Dysregulation of actin-bundling proteins, including fimbrin, can affect cytoskeletal organization and has been studied in contexts related to cell migration, immune function, and other physiological processes.