Multifunctional actin-bundling protein. Plays a major role in regulating the organization, dimension, dynamics and signaling capacities of the actin filament-rich microvilli in the mechanosensory and chemosensory cells (PubMed:9763424). Required for the assembly and stabilization of the stereociliary parallel actin bundles. Plays a crucial role in the formation and maintenance of inner ear hair cell stereocilia. Involved in the elongation of actin in stereocilia. In extrastriolar hair cells, required for targeting MYO3B to stereocilia tips, and for regulation of stereocilia diameter and staircase formation (By similarity). Monomer (By similarity). Interacts with PFN2 (By similarity). Binds F-actin in a Ca(2+)-resistant fashion (PubMed:8799813, PubMed:9763424). Interacts (via N-terminal) with BAIAP2 (via SH3-domain) (PubMed:12598619). Interacts with MYO3A (via C-terminus). Interacts with MYO3B (via C-terminus) (By similarity). Isoform 1 localizes to parallel actin bundles of ectoplasmic specializations between neighboring Sertoli cells and at sites where Sertoli cells contact the heads of elongate spermatids. Isoform 2 localizes to parallel actin bundles of brush border microvilli of small intestine and kidney. Expressed at high concentration in the microvillar parallel actin bundle (PAB) of hair cells stereocilia in the cochlea and vestibular system. Detected also at high levels of a number of other sensory cell types, including taste receptor cells, solitary chemoreceptor cells, vomeronasal sensory neurons and Merkel cells. Isoform 1 is detected in testis. Isoforms 2 is detected in small intestine and kidney (at protein level). Isoforms 3, 4, 6 and 8 are expressed in Purkinje cells dendritic spines. Isoform 2 accumulates in the brush border during enterocyte differentiation and migration along the crypt-villus axis in adults. The WH2-domain binds actin monomer and mediated actin bundle assembly.