Retina-Specific Protein Fascin 2 Is an Actin Cross-linker Associated with Actin Bundles in Photoreceptor Inner Segments and Calycal Processes

Abstract

Fascin 2 is a retinal-specific member of the fascin family of actin filament-bundling proteins. Fascin 2 mutation in humans results in autosomal dominant retinitis pigmentosa or macular degeneration. To investigate the role of fascin 2 in photoreceptor survival, the authors examined its localization in photoreceptors and characterized its interactions with actin filaments in vitro. Fascin 2 localization was determined by immunohistochemistry and transgenic expression of green fluorescent protein (GFP)-tagged fascin 2 in Xenopus laevis rods. Fascin 2 actin-binding and actin-bundling activity were examined in sedimentation assays using bacterially expressed fusion proteins and polymerized actin. To assess the role of phosphorylation of a conserved serine (amino acid 39) in fascin 2 on subcellular localization and actin-binding, effects of serine mutants were also examined in transgenic Xenopus and in in vitro assays. Fascin 2 is localized to actin filament bundles of the photoreceptor inner segment and calycal processes. Like fascin 1, fascin 2 binds and cross-links actin filaments. Mutation of serine 39 to an aspartic acid reduced fascin 2 binding of actin filaments and abolished fascin 2 bundling of actin filaments in vitro but produced no detectable effect on GFP-tagged fascin 2 localization in transgenic Xenopus. These observations suggest that fascin 2 plays a role in the assembly or stabilization of inner segment and calycal process actin filament bundles in photoreceptors and that serine 39 phosphorylation reduces actin-binding and cross-linking activity and, thus, is likely to regulate the inner segment actin cytoskeleton.