The early steps in vertebrate vision require fast interactions between Rh (rhodopsin) andG t (transducin), which are classically described by a collisional coupling mechanism driven by the free diffusion of monomeric proteins on the disc membranes of rod and cone cells. Recent findings, however, point to a very low mobility for Rh and support a substantially different supramolecular organization. Moreover, Rh-G t interactions seem to possibly occur even prior to light stimuli, which is also difficult to reconcile with the classical scenario. We investigated the kinetics of interaction between native Rh and G t in different conditions by surface plasmon resonance and analysed the results in the general physiological context by employing a holistic systems modelling approach. The results from the present study point to amechanism that is intermediate between pure collisional coupling and physical scaffolding. Such a 'dynamic scaffolding', in which prevalently dimeric Rh and Gt interact in the dark by forming transient complexes (∼25% of G t is precoupled to Rh), does not slow down the phototransduction cascade, but is compatible with the observed photoresponses on a broad scale of light stimuli. We conclude that Rh molecules and Rh-G tcomplexes can both absorb photons and trigger the visual cascade.
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