Distinctive subtypes of bovine phospholipase C that have preferential expression in the retina and high homology to the norpA gene product of Drosophila.

The Drosophila norpA gene encodes a phospholipase C involved in phototransduction. However, phospholipase C apparently is not directly involved in phototransduction in vertebrate photoreceptors, although light-activated phospholipase C activity has been reported in vertebrate rod outer segments. Conserved regions of norpA cDNA were used to isolate bovine cDNAs that would encode four alternative forms of phospholipase C of the beta class that are highly homologous to the norpA protein and expressed preferentially in the retina. Two of the variants are highly unusual in that they lack much of the N-terminal region present in all other known phospholipases C. The sequence conservation between these proteins and the norpA protein is higher than that between any other known phospholipases C. GTPase sequence motifs found in proteins of the GTPase superfamily are found conserved in all four variants of the bovine retinal protein as well as the norpA protein but not in other phospholipases C. Results suggest that these proteins together with the norpA protein constitute a distinctive subfamily of phospholipases C that are closely related in structure, function, and tissue distribution. Mutations in the norpA gene, in addition to blocking phototransduction, cause light-dependent degeneration of photoreceptors. In view of the strong similarity in structure and tissue distribution, a defect in these proteins may have similar consequences in the mammalian retina.