Size changes of phosphodiesterase in bovine rod outer segments on illumination

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Abstract
Light activates a cGMP phosphodiesterase (PDE) in bovine retinal rod outer segments. The light is absorbed by rhodopsin situated in the disk membranes. PDE is a 3 subunit peripheral protein on the disks and appears to be activated via a guanine nucleotide binding protein (G) in the presence of activated rhodopsin and GTP. The activation may occur by collision coupling of G and PDE. The protein-protein interactions of PDE was studied in situ in disk membranes by radiation inactivation. Irradiation of a protein with high-energy electrons leads to loss of activity in proportion to radiation dose and the molecular weight of the protein. No change was seen in the size of PDE upon activation by light and 100 .mu.M guanosine 5''-(.beta.,.gamma.-imidotriphosphate) (Gpp[NH]p) compared with PDE in dark with 260 .mu.M GTP. Application of statistics to shows that a 27,000 change in MW would be significant at the 95% level but that smaller changes would go undetected. The apparent MW is 176,000 .+-. 27,000 (mean .+-. 95% confidence limit), in agreement with the size determined by polyacrylamide gel electrophoresis. There appears to be either no permanent change in PDE size on activation or a small change, undetectable by the technique, or an exchange of subunits such that no net change in MW is seen.