Abstract
Visual phototransduction begins with photon capture by rhodopsin. Dense packing of rhodopsins in the outer segment membranes improves photon capture but can hinder the photoresponse by impeding the lateral diffusion of transduction proteins on the membrane. We attempted to investigate this effect by over-expressing rhodopsin in rods of transgenic mice. Increased rhodopsin expression was confirmed by Western analyses and by single cell microspectrophotometry. However, electron microscopy revealed that the excessive rhodopsins did not increase the packing density. Rods simply expanded the size of their membranous disks to accommodate the extra rhodopsins without increasing membrane congestion. Rod sensitivity increased due to improved photon capture. The dispersal of transducin and phosphodiesterase within the more spacious outer segment and the greater interdiskal volume delayed the photoresponse onset and reduced the cascade amplification. Flash responses from mutant and wild-type rods were fit with a mathematical model that adjusted for the alteration in outer segment structure.
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