Light- and GTP-activated hydrolysis of phosphatidylinositol bisphosphate in squid photoreceptor membranes
Light stimulates the hydrolysis of exogenous, (/sup 3/H)inositol-labeled phosphatidylinositol bisphosphate (PtdInsP2) added to squid photoreceptor membranes, releasing inositol trisphosphate (InsP3). At free calcium levels of 0.05 microM or greater, hydrolysis of the labeled lipid is stimulated up to 4-fold by GTP and light together, but not separately. This activity is the biochemical counterpart of observations on intact retina showing that a rhodopsin-activated GTP-binding protein is involved in visual transduction in invertebrates, and that InsP3 release is correlated with visual excitation and adaptation. Using an in vitro assay, we investigated the calcium and GTP dependence of the phospholipase activity. At calcium concentrations between 0.1 and 0.5 microM, some hydrolysis occurs independently of GTP and light, with a light- and GTP-activated component superimposed. At 1 microM calcium there is no background activity, and hydrolysis absolutely requires both GTP and light. Ion exchange chromatography on Dowex 1 (formate form) of the water-soluble products released at 1 microM calcium reveals that the product is almost entirely InsP3. Invertebrate rhodopsin is homologous in sequence and function to vertebrate visual pigment, which modulates the concentration of cyclic GMP through the mediation of the GTP-binding protein transducin. While there is some evidence that light also modulates PtdInsP2 content in vertebrate photoreceptors, the case for its involvement in phototransduction is stronger for the invertebrate systems. The results reported here support the scheme of rhodopsin----GTP-binding protein----phospholipase C activation in invertebrate photoreceptors.
- Research Organization:
- Univ. of Oxford, England
- OSTI ID:
- 5600619
- Journal Information:
- J. Biol. Chem.; (United States), Journal Name: J. Biol. Chem.; (United States) Vol. 263:1; ISSN JBCHA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
59 BASIC BIOLOGICAL SCIENCES
ANIMALS
AQUATIC ORGANISMS
BIOCHEMICAL REACTION KINETICS
CARBOXYLESTERASES
CELL CONSTITUENTS
CELL MEMBRANES
CHEMICAL REACTIONS
DECOMPOSITION
ELECTROMAGNETIC RADIATION
ENZYME ACTIVITY
ENZYMES
ESTERASES
ESTERS
GUANYLIC ACID
HYDROLASES
HYDROLYSIS
INVERTEBRATES
ISOTOPE APPLICATIONS
KINETICS
LABELLED COMPOUNDS
LIPASE
LIPIDS
LYSIS
MEMBRANES
METABOLISM
MOLLUSCS
NUCLEOTIDES
ORGANIC COMPOUNDS
ORGANIC PHOSPHORUS COMPOUNDS
PHOSPHOLIPIDS
PIGMENTS
PROTEINS
RADIATIONS
REACTION KINETICS
RHODOPSIN
SOLVOLYSIS
TRACER TECHNIQUES
TRITIUM COMPOUNDS
VISIBLE RADIATION