Reversible phosphorylation of opsin induced by irradiation of blowfly retinae

Light-induced phosphorylation and dephosphorylation of the visual pigment protein, opsin, was investigated in isolated retinae of the blowfly making use of the fact that photon capture by rhodopsin leads to the formation of a thermostable metarhodopsin. It is proposed that the phosphorylated forms of rhodopsin and metarhodopsin represent inactive states of the pigment, i.e. phosphorylated metarhodopsin does not initiate reactions leading to the excitation of the photoreceptor cell and phosphorylated rhodopsin cannot be converted into physiologically active metarhodopsin without first being dephosphorylated.