Transient transfection assays were used to determine how the activity of phospholipase C beta 4, which is preferentially expressed in retina, is regulated. All the alpha subunits of the Gq class, including G alpha q, G alpha 11, G alpha 14, G alpha 15, and G alpha 16 can activate PLC beta 4 and none of the G beta gamma subunits tested, including G beta 1 gamma 1, G beta 1 gamma 2, G beta 1 gamma 3, or G beta 2 gamma 2, activate phospholipase C beta 4. In control experiments, cotransfection with cDNA encoding the alpha subunit of transducin or Gi2 gives no activation of PLC beta 4. Thus, deoxyribonucleotides and ribose 5-phosphate are not inhibitory. The monophosphate, diphosphate, and triphosphate nucleoside derivatives are all inhibitory, whereas cyclic nucleotides are ineffective.
Purine nucleotides are more potent inhibitors than pyrimidine nucleotides.
However, no direct role for a phospholipase C in vertebrate phototransduction has been identified to date. Bovine cDNAs encoding PI-PLC isoforms expressed predominantly in the retina have been isolated and characterized. Rat PLC beta 4 cDNA is highly homologous to the norpA cDNA of Drosophila . PLC beta 4 gene expression has been mapped in rat brain tissue sections by in situ hybridization. The PLC beta 4 gene is expressed at high abundance in cerebellar Purkinje cells and neurones of the substantia nigra, the median geniculate bodies and the thalamic nuclei.