Binding and activation of rod outer segment phosphodiesterase and guanosine triphosphate binding protein by disc membranes: influence of reassociation method and divalent cations

Attempts to optimize the recovery of light-stimulated phosphodiesterase activity following reassociation of the hypotonically extractable proteins derived from retinal rod segments with hypotonically stripped disc membranes lead to the following observations: the best reassociations were obtained by mixing proteins and stripped disc membranes under hypotonic conditions and slowly increasing the salt concentration; the binding of G-protein and phosphodiesterase to stripped disc membrane occurs in less than 5 minutes and the recovery of light-stimulated phosphodiesterase activation in response to subsaturating stimulus levels requires 2-3 h to plateau. Stripped disc membranes and proteins were reassociated in 'isotonic' buffers containing KCl/NaCl, KCl/NaCl plus Mg2+, or KCl/NaCl plus Ca2+. Large fractional rhodopsin bleaches produced nearly identical light-stimulated phosphodiesterase activities in each of these samples and in the control rod outer segment membranes. Rod outer segment membranes and reassociated stripped disc membrane samples containing divalent cations showed similar phosphodiesterase activities in response to low fractional rhodopsin bleaches (e.g. less than or equal to 0.1%), however, samples devoid of divalent cations during reassociation required rhodopsin bleaches up to 10-fold larger to elicit comparable phosphodiesterase activities. These results suggest that not all phosphodiesterase and/or G-protein molecules bound to the disc membrane surface are equivalent with regard to their efficiency of activation by bleached rhodopsin and that divalent cations can modulate the distribution of G-protein and/or phosphodiesterase between these populations.