Expression of usherin in the anthozoan Nematostella vectensis

The transmembrane splice variant of usherin is concentrated in the ankle-link complex of stereocilia in the vertebrate cochlea. Mutations in this variant can result in Usher Syndrome, or congenital deafblindness. In order to study the phylogenetically conserved features of this protein, two adjacent putative genes encoding predicted proteins similar to the amino and carboxy halves of vertebrate usherin were identified in the genome of the anthozoan Nematostella vectensis. RT-PCR was used to show that transcripts encoding each predicted protein are expressed during development, and that both predicted proteins are in fact encoded on the same gene. Thus, anthozoan usherin has a domain organization identical to the transmembrane variant of vertebrate usherin, with a laminin N-terminal domain, 10 epidermal growth factor-like domains, four fibronectin-type III domains, two laminin G domains, 28 additional fibronectin-type III domains, a transmembrane domain, and a short intracellular domain. Usherin is expressed widely in planulae and in the ectoderm of the column of juvenile polyps. Searches of genomic databases reveal usherins with a highly conserved domain organization throughout the deuterostome lineage as well as in some (annelids and molluscs), but not all (arthropods and nematodes), protostomes. Thus, usherin sequence and domain organization is remarkably conserved across phyla. However, its pattern of expression in N. vectensis is not consistent with a role for this protein in the mechanosensory ciliary cone of mature cnidoblasts.