Signaling States of Rhodopsin in Rod Disk Membranes Lacking Transducin βγ-Complex

PURPOSE. To characterize the possible role of transducin Gt beta gamma-complex in modulating the signaling properties of photoactivated rhodopsin and its lifetime in rod disc membranes and intact rods. METHODS. Rhodopsin photolysis was studied using UV-visible spectroscopy and rapid scanning spectroscopy in the presence of hydroxylamine in highly purified wild-type and Gt gamma-deficient mouse rod disc membranes. Complex formation between photoactivated rhodopsin and transducin was measured by extra-metarhodopsin (meta) II assay. Recovery of dark current and flash sensitivity in individual intact wild-type and Gt gamma-deficient mouse rods was measured by single-cell suction recordings. RESULTS. Photoconversion of rhodopsin to meta I/meta II equilibrium proceeds normally after elimination of the Gt beta gamma-complex. The meta I/meta II ratio, the rate of meta II decay, the reactivity of meta II toward hydroxylamine, and the rate of meta III formation in Gt gamma-deficient rod disc membranes were identical with those observed in wild-type samples. Under low-intensity illumination, the amount of extra-meta II in Gt gamma-deficient discs was significantly reduced. The initial rate of dark current recovery after 12% rhodopsin bleach was three times faster in Gt gamma-deficient rods, whereas the rate of the late current recovery was largely unchanged. Mutant rods also exhibited faster postbleach recovery of flash sensitivity. CONCLUSIONS. Photoactivation and thermal decay of rhodopsin proceed similarly in wild-type and Gt gamma-deficient mouse rods, but the complex formation between photoactivated rhodopsin and transducin is severely compromised in the absence of Gt beta gamma. The resultant lower transduction activation contributes to faster photoresponse recovery after a moderate pigment bleach in Gt gamma-deficient rods. (Invest Ophthalmol Vis Sci. 2012;53:1225-1233) DOI:10.1167/iovs.11-9350