A2-rhodopsin: a new fluorophore isolated from photoreceptor outer segments.

A2E and iso-A2E are fluorescent amphiphilic pyridinium bisretinoids involved in age-related macular degeneration (AMD). It is now shown that the presence of high exogenous concentrations of all-trans-retinal in photoreceptor outer segments leads to the formation of A2-rhodopsin (A2-Rh), an unprecedented fluorescent rhodopsin adduct which consists of bisretinoids (A2) linked to each of three lysine residues in rhodopsin (Rh) and which exhibits an emission spectrum similar to A2E. The fluorophore to protein ratio was determined by MALDI-TOF-MS and UV-VIS spectroscopy. Enzymatic degradation with thermolysin and cathepsin D showed that two of the A2 moieties were located in the region of the third cytoplasmic loop and 8th helix of Rh. Examination of A2-Rh and A2-PE (the precursor of A2E) fluorescence in relation to all-trans-retinal concentration indicated that whereas A2-PE formation is favored over that of A2-Rh, for a single rhodopsin molecule only one phosphatidylethanolamine molecule is available to react with all-trans-retinal; this phosphatidylethanolamine is probably tightly associated with the protein.