Retina-specific protein fascin 2 is an actin cross-linker associated with actin bundles in photoreceptor inner segments and calycal processes.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Lin-Jones
Jennifer
J
Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California, USA. linjones@berkeley.edu
Burnside
Beth
B
D008841:Actin Cytoskeleton / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified; D015153:Blotting, Western; D003001:Cloning, Molecular; D005136:Eye Proteins / Q000737:chemistry / Q000378:metabolism*; D015870:Gene Expression; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D008840:Microfilament Proteins / Q000737:chemistry / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D010766:Phosphorylation; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism*; D011993:Recombinant Fusion Proteins; D012694:Serine / Q000378:metabolism; D019076:Transgenes; D014982:Xenopus laevis; D015027:Zebrafish
D005136:Eye Proteins; C518610:FSCN2 protein, Xenopus; D008840:Microfilament Proteins; D011993:Recombinant Fusion Proteins; C518611:fscn2b protein, zebrafish; D049452:Green Fluorescent Proteins; D012694:Serine