Sorting of actin isoforms in chicken auditory hair cells

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Abstract
Most nonmuscle cells of higher vertebrates contain two different actin isoforms, beta- and gamma-cytoplasmic actin. The beta-isoform is with few exceptions the predominant isoform in nonmuscle cells and tissues. Perturbation of the beta:gamma ratio has been shown to affect the organization of bundled actin filaments indicating that the beta- and gamma-genes encode functionally distinct cytoarchitectural information. In the present study we localized by immunostaining beta- and gamma-actin in chicken auditory hair cells. These highly specialized cells serve as model system for studying certain developmental and structural aspects of a complex actin filament system with high architectural precision. We show that gamma-actin is the predominant actin isoform in auditory hair cells with an apparent beta:gamma ratio of approximately 1:2. gamma-Actin is not sorted and occurs in all three actin assemblies of the hair border, i.e. the cores of sensory hairs (stereocilia), the subjacent gel-like actin filament meshwork (cuticular plate) and the zonula adherens ring. In contrast to gamma-actin, the beta-isoform is specifically sorted to the actin filament core bundle of stereocilia that is extensively crosslinked by fimbrin. In view of recent studies showing that L-plastin, the leukocyte homolog of fimbrin, has a higher binding affinity for beta-actin than for gamma-actin, a mechanism is proposed for how hair cells might restrict formation of actin filament bundles to a single cellular site (i.e. the stereocilia). The limited level of expression of beta-actin in hair cells may help to prevent ectopic bundle formation in other cellular compartments.