PHR1 encodes an abundant, pleckstrin homology domain-containing integral membrane protein in the photoreceptor outer segments

We cloned human and murine cDNAs of a gene (designated PHR1), expressed pre ferentially in retina and brain. In both species, PHR1 utilizes two promote rs and alternative splicing to produce four PHR1 transcripts, encoding isof orms of 243, 224, 208, and 189 amino acids, each with a pleckstrin homology domain at their N terminus and a transmembrane domain at their C terminus. Transcript 1 originates from a 5'-photoreceptor-specific promoter with at least three Crx elements ((C/T)TAATCC). Transcript 2 originates from the sa me promoter but lacks exon 7, which encodes 35 amino acids immediately C-te rminal to the pleckstrin homology domain. Transcripts 3 and 4 originate fro m an internal promoter in intron 2 and either include or lack exon 7, respe ctively. In situ hybridization shows that PHR1 is highly expressed in photo receptors, with lower expression in retinal ganglion cells. Immunohistochem istry localizes the PHR1 protein to photoreceptor outer segments where chem ical extraction studies confirm it is an integral membrane protein. Using a series of PHR1 glutathione S-transferase fusion proteins to perform in vit ro binding assays, we found PHR1 binds transducin beta gamma subunits but n ot inositol phosphates. This activity and subcellular location suggests tha t PHR1 may function as a previously unrecognized modulator of the phototran sduction pathway.