C-terminal threonines and serines play distinct roles in the desensitization of rhodopsin, a G protein-coupled receptor.

Rod photoreceptors generate measurable responses to single-photon activation of individual molecules of the G protein-coupled receptor (GPCR), rhodopsin. Timely rhodopsin desensitization depends on phosphorylation and arrestin binding, which quenches G protein activation. Rhodopsin phosphorylation has been measured biochemically at C-terminal serine residues, suggesting that these residues are critical for producing fast, low-noise responses. The role of native threonine residues is unclear. We compared single-photon responses from rhodopsin lacking native serine or threonine phosphorylation sites. Contrary to expectation, serine-only rhodopsin generated prolonged step-like single-photon responses that terminated abruptly and randomly, whereas threonine-only rhodopsin generated responses that were only modestly slower than normal. We show that the step-like responses of serine-only rhodopsin reflect slow and stochastic arrestin binding. Thus, threonine sites play a privileged role in promoting timely arrestin binding and rhodopsin desensitization. Similar coordination of phosphorylation and arrestin binding may more generally permit tight control of the duration of GPCR activity.

A survey of molecular expression by photoreceptors after experimental retinal detachment.

PURPOSE: To describe changes in the localization patterns and levels of rod and cone photoreceptor proteins after experimental retinal detachment (RD). METHODS: Cat retinas were detached for 1, 3, 7, or 28 days, at which time the eyecups were placed in fixative for immunocytochemical analysis or homogenized for biochemistry. Immunocytochemistry was performed using 19 probes for molecules known to be associated with photoreceptors. Protein concentrations were determined using enzyme-linked immunosorbent assay or Western blot analysis. Cone cell death was analyzed by double labeling with TdT-dUTP terminal nick-end labeling and cone-specific antibodies. RESULTS: Although some cones died, many survived long-term RD. Although their profiles may have changed, rod photoreceptors continued to express most of the molecules studied as long as they were alive. In contrast, the cones failed to label with almost all probes specific to them after 3 to 7 days of detachment. The exception was phosducin, which localized to both rods and cones and, in 28-day detachments, increased to 180% of the amount in normal retina. CONCLUSIONS: Rods and cones respond differently to RD. This difference may account for a faster return of rod vision and for the lingering changes in color vision and acuity that are often reported after successful reattachment surgeries.