Rat retina has an active and stable ubiquitin–protein conjugating system

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Abstract
We describe here the presence of ubiquitin and its conjugation system in the rat retina. Retinal homogenates and supernatants conjugate [125I] human ubiquitin with either endogenous or exogenous proteins. The conjugating activity is relatively stable over time, requires ATP, and has a pH optimum of ∼8. The most prominent [125I]ubiquitin conjugates formed are larger than 130 kDa. Several other minor conjugates are also formed between the molecular weights 17 and 75 kDa. The endogenous levels of free and conjugated forms of ubiquitin have been determined in the rat retina. More than 50% of retinal ubiquitin is covalently bound to target proteins. In addition, activities responsible for the ATP‐dependent degradation and disassembly of both endogenous and exogenous ubiquitin conjugates have been detected in vitro. These results provide evidence that the retina contains active and stable ubiquitin‐conjugating enzymes that recognize retinal proteins and have ATP‐dependent proteolytic activity.