Regulation of protein phosphatase‐1_G from rabbit skeletal muscle:2. Catalytic subunit translocation is a mechanism for reversible inhibition of activity toward glycogen‐bound substrates

The glycogen‐associated form of protein phosphatase‐1 (PP‐1G) comprises a 37‐kDa catalytic (C) subunit and a 161‐kDa glycogen‐binding (G) subunit. In the preceding paper in this issue of the journal we showed that the C subunit is released from PP‐1G in response to phosphorylation of the G subunit by cAMP‐dependent protein kinase. We now show that at 0.15–02 M KCl the phosphorylase phosphatase activity of glycogen‐bound PP‐1G is 5–8 times higher than that of released C subunit or unbound PP‐1G, which are strongly inhibited at these ionic strengths. The activity of glycogen‐bound PP‐1G towards glycogen synthase was about 5‐fold higher than that of released C subunit at 0.15M KCl. Studies with glycogen‐bound substrates and myosin P‐light chain (which does not interact with glycogen) indicated that PP‐1G activity is only enhanced compared to free C subunit at near physiological ionic strength and when both PP‐1G and substrate are glycogen‐associated. The inhibition by increasing ionic strength and enhanced activity upon binding to glycogen reflected changes in K′m, but not Vmax. From the determined specificity constant, k′cat/K′m∼ 4 × 106 s−1 M−1, it was calculated that at physiological levels of glycogen‐bound PP‐1G (200 nM) and phosphorylase (70 μM). dephosphorylation of the latter could occur with a half time of 15 s, sufficient to account for inactivation rates in vivo. The much higher catalytic efficiency of glycogen‐bound PP‐1G toward the glycogen‐metabolising enzymes at physiological ionic strength compared to free C subunit substantiates the role of PP‐1G in the regulation of these substrates, and establishes a novel mechanism for selectively regulating their phosphorylation states in response to adrenalin and other factors affecting phosphorylation of the G subunit.