Analysis of a vinculin homolog in a sponge (phylum Porifera) reveals that vertebrate-like cell adhesions emerged early in animal evolution.
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VIN1 is a vinculin ortholog by phylogeny and structure but lacks domain D2.
"However, Op vinculin lacks domain 2, which typically distinguishes vinculin from α-catenin in other animals"
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Recombinant VIN1 binds a sponge talin vinculin-binding-site peptide but not a sponge beta-catenin peptide (ITC).
"Moreover, Op vinculin bound talin but not β-catenin"
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VIN1 alone does not bind F-actin; talin peptide activates F-actin binding (co-sedimentation).
"Thus, binding of talin to Op vinculin D1 releases autoinhibition for F-actin binding, similar to the behavior of vertebrate vinculin."
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Endogenous VIN1, detected with a validated antibody in fixed sponge tissue, co-localizes with cortical actin belts at cell-cell contacts in pinacoderm and choanoderm and with actin in filopodia of migratory mesohyl cells.
"Immunofluorescence of O. pearsei tissue using an affinity-purified antibody raised against Op vinculin showed co-localization with cortical belts of actin filaments at cell–cell contacts in the pinacoderm and choanoderm tissues"
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The cell-cell contacts at which VIN1 localizes are not ultrastructurally defined junctions.
"The localization of Op vinculin to cell–cell contacts in the pinacoderm and choanoderm is consistent with a possible AJ in these tissues, despite the lack of ultrastructural evidence"