In vivo light exposure results in tyrosine phosphorylation of several rod outer segment (ROS) proteins (Ghalayini, A. J., Guo, X. X., Koutz, C. A, and Anderson, R. E. (1998) Exp. Eye Res. 66, 817-821). We now report the presence of Src in ROS and its increased association with bleached ROS membranes. Immunoprecipitation with anti-phosphotyrosine revealed that tyrosine kinase activity recovered from light-adapted ROS membranes was twice that recovered from dark-adapted ROS. Other experiments revealed the presence of both bleached rhodopsin and arrestin in immunoprecipitates of LROS, suggesting the formation of a multimeric complex containing Src, arrestin, and bleached rhodopsin. Additionally, when immobilized Src homology domains 2 and 3 (SH2 and SH3, respectively) were used to study the association of Src with ROS membranes, only bleached opsin and arrestin were found to associate with the SH2 domain of Src. These data strongly suggest that Src through its SH2 domain interacts with bleached rhodopsin and arrestin either directly or indirectly. Similar results were also obtained when dark-adapted and light-adapted retinas were used instead of ROS membranes. Our data strongly suggest that light exposure in vivo activates Src and promotes its association through its SH2 domain with a complex containing bleached rhodopsin and arrestin. A hypothesis for the functional significance of this phenomenon is presented.Tyrosine phosphorylation plays an important role in many cellular functions including growth, development, proliferation, and survival. Several extracellular signals including insulin, growth factors, mitogens, and others produce their intracellular effects through the activation of the appropriate receptor tyrosine kinases or nonreceptor protein-tyrosine kinases (1). This activation results in tyrosine phosphorylation of key intracellular proteins that are either enzymes or play the role of adaptor proteins between other intracellular signaling proteins (2). In the case of receptor tyrosine kinases, receptor activation causes a rapid autophosphorylation on tyrosine residues, which leads to association of these receptors with key intracellular proteins, resulting in tyrosine phosphorylation of these target proteins. Among the enzymes that are tyrosinephosphorylated upon activation are Ras-GTPase-activating protein, phospholipase C␥ (PLC␥), 1 phosphatidylinositol 3-kinase, protein-tyrosine phosphatases including the ubiquitous SHP-2, and a growing list of other enzymes (2-5).Although most of the current information regarding the biological significance of tyrosine phosphorylation involves receptor tyrosine kinases and nonreceptor protein-tyrosine kinases that are stimulated by growth factors and mitogens, there is recent evidence indicating the involvement of G-protein-coupled receptors (GPCRs) in the generation of tyrosine phosphorylation-based signals (for a recent review, see Ref. 6). One such example is the angiotensin II receptor (AT 1 ), a seventransmembrane receptor with no intrinsic kinase activity, w...

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