CRB1 has a cytoplasmic domain that is functionally conserved between human and Drosophila

Mutations in the human Crumbs homologue 1 (CRB1) gene cause severe retinal dystrophies, ranging from retinitis pigmentosa to Leber congenital amaurosis. The CRB1 gene is expressed specifically in human retina and brain and encodes a protein homologous to the Drosophila Crumbs protein. In crumbs mutant embryos apico-basal polarity of epithelial cells is lost, leading to widespread epidermal cell death. The small cytoplasmic domain of Crumbs organizes an intracellular protein scaffold that defines the assembly of a continuous zonula adherens. The crumbs mutant phenotype can be partially rescued by expression of just the membrane-bound cytoplasmic domain, and overexpression of this domain in a wild-type background results in a multilayered epidermis. A striking difference between CRB1 and Crumbs was that the latter contains a transmembrane region and a 37 amino acid cytoplasmic domain. Here we describe an alternative splice variant of human CRB1 that encodes a cytoplasmic domain 72% similar to that of Drosophila Crumbs. Two intracellular subdomains that are necessary for function in Drosophila are absolutely conserved. Rescuing and overexpression studies in Drosophila show that the cytoplasmic domains are functionally related between these distant species. This suggests that CRB1 organizes an intracellular protein scaffold in the human retina. Human homologues of proteins binding to Crumbs may be part of this complex and represent candidate genes for retinal dystrophies.