Interplay of Phosphorylation and Dephosphorylation in Vision: Protein Phosphatases of Bovine Rod Outer Segments

Charles Fowles, Muhammad Akhtar, Philip Cohen

    Research output: Contribution to journalArticle

    53 Citations (Scopus)

    Abstract

    Two types of protein phosphatases were identified in carefully prepared bovine rod outer segments (ROS). Extraction of the ROS with a medium-salt buffer solubilized protein phosphatase activity that was mainly type 2A, since it was active toward phosphorylase a in the absence of divalent cations, was not retained by heparin-Sepharose, dephosphorylated the α-subunit of phosphorylase kinase faster that the β-subunit, and was unaffected by inhibitor 2. Further extraction of the resulting membranes with a high-salt buffer solubilized additional phosphatase activity which was predominantly type 1,since it was retained by heparin-Sepharose and was blocked by inhibitor 2. The molecular mass of the type 2A phosphatase estimated by gel permeation chromatography on Superose 12 was 100 kDa,suggesting it may be the 2A2 form. Only the ROS type 2A phosphatase dephosphorylated opsin and rhodopsin efficiently. Concordant with this finding, the purified catalytic subunit of protein phosphatase 2A from rabbit skeletal muscle dephosphorylated opsin efficiently, while the type 1 catalytic subunit isolated from this tissue was inactive. Together, the results suggest that the ROS type 2A protein phosphatase plays an important role in regenerating rhodopsin from the various phosphorylated species in vivo. The activity of the enzyme per retina (~ 85 pmol of Pi released/min) is comparable to that of rhodopsin kinase (100 pmol of phosphate transferred/min).

    Original languageEnglish
    Pages (from-to)9385-9391
    Number of pages7
    JournalBiochemistry
    Volume28
    Issue number24
    DOIs
    Publication statusPublished - 1989

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