Opsin/all-trans-retinal complex activates transducin by different mechanisms than photolyzed rhodopsin

Biochemistry. 1996 Mar 5;35(9):2901-8. doi: 10.1021/bi9524068.


In rhodopsin, the 11-cis-retinal chromophore forms a complex with Lys296 of opsin via a protonated Schiff base. Absorption of light initiates the activation of rhodopsin by cis/trans photoisomerization of retinal. Thermal relaxation through different intermediates leads into the metarhodopsin states which bind and activate transducin (Gt) and rhodopsin kinase (RK). all-trans-Retinal also recombines with opsin independent of light, forming activating species of the receptor. In this study, we examined the mechanism by which all-trans-retinal activates opsin. To exclude other amines except active site Lys296 from formation of Schiff bases, we reductively methylated rhodopsin (PM-rhodopsin), which we then bleached to generate PM-opsin. Using spectroscopic methods and a Gt activation assay, we found that all-trans-retinal interacted with PM-opsin, producing a noncovalent complex that activated Gt. The residual nucleotide exchange in Gt catalyzed by opsin was approximately 1/250 lower relative to that of photoactivated rhodopsin (pH 8.0, 23 degrees C). Addition of equimolar all-trans-retinal led to an occupancy of one-tenth of the putative retinal binding site(s) of opsin and enhanced the Gt activation rate 2-fold. When the concentration of all-trans-retinal was increased to saturation, the Gt activation rate of the opsin/all-trans-retinal complex was approximately 1/33 lower compared to that of photoactivated rhodopsin. We conclude that all-trans-retinal can form a noncovalent complex with opsin that activates Gt by different mechanisms than photolyzed rhodopsin.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Binding Sites
  • Cattle
  • Darkness
  • GTP-Binding Proteins / metabolism*
  • Hydrogen-Ion Concentration
  • Kinetics
  • Light
  • Lysine
  • Methylation
  • Photolysis
  • Retinaldehyde / pharmacology*
  • Rhodopsin / metabolism*
  • Rod Cell Outer Segment / metabolism*
  • Rod Opsins / metabolism*
  • Schiff Bases
  • Spectrophotometry
  • Stereoisomerism
  • Transducin / metabolism*


  • Rod Opsins
  • Schiff Bases
  • Rhodopsin
  • GTP-Binding Proteins
  • Transducin
  • Lysine
  • Retinaldehyde