The protein phosphatases involved in cellular regulation

Abstract

The effects of polyamines on the oligomeric forms of protein phosphatase-1 (IG), protein phosphatase-2A (2Ao, 2A1 and 2Az) and their free catalytic subunits (Ic and 2Ac) has been studied using homogeneous enzymes isolated from rabbit skeletal muscle. Spermine increased the activity of protein phosphatase-2A towards eight of nine substrates tested. Half-maximal activation was observed at 0.2 mM with optimal effects at 1 -2 mM. Above 2 mM, spermine became inhibitory. The most impressive activation of protein phosphatase-2A was obtained with glycogen synthase, especially when phosphorylated at sites-3 (8 -15-fold with protein phosphatase-2A and phenylalanine hydroxylase (6 -7-fold with protein phosphatase-2Al) as substrates. Activation of protein phosphatases 2Ao, 2A1 and 2A2 was greater than that observed with 2Ac. Spermine was a more potent activator than spermidine, while putrescine had only a small effect. Qualitatively similar results were obtained with five other substrates, although maximal activation was much less (1.3 -3-fold with protein phosphatase-2Al). The rate of dephosphorylation of glycogen phosphorylase was decreased by spermine, inhibition being more pronounced with protein phosphatase-2Ac than with 2Ao, 2A1 and 2A2. Spermine (I50 = 0.1 mM with protein phosphatase-2Ac) was a more potent inhibitor than spermidine (Zs0 = 0.9 mM) or putrescine (Iso = 8 mM). Partially purified preparations of protein phosphatases-2Ao, 2A1 and 2Az from from rat liver were affected by spermine in a similar manner to the homogeneous enzymes from rabbit skeletal muscle.Spermine did not activate protein phosphatase-1 to the same extent as protein phosphatase-2A. Greatest stimulation (2.5-fold) was again observed with glycogen synthase labelled in sites-3, with half-maximal activation at 0.2 mM and optimal effects at 1-2 mM spermine. Spermine was a much more effective stimulator than spermidine, while putrescine was ineffective. Very similar results were obtained with protein phosphatases lG and lc. With four other substrates maximal activation by spermine was < 1.5-fold, while the dephosphorylation of glycogen synthase (labelled in site-2), phosphorylase kinase, pyruvate kinase and glycogen phosphorylase were inhibited. Spermine ( I 5 0 = 0.04mM) was a more potent inhibitor of the dephosphorylation of glycogen phosphorylase than spermidine (I50 = 0.9 mM) or putrescine (I50 = 9 mM).There was some resemblance between the effects of Mn2 + and spermine on the activity of protein phosphatase-2A, although with the substrates inhibitor-1 and phenylalanine hydroxylase, activation by Mn2 ' was greater than that observed with spermine, while with glycogen synthase the converse was true. MnZ' could not mimic the activating effect of spermine on dephosphorylation of glycogen synthase (labelled in sites-3) by protein phosphatase 1. By contrast, spermine hardly affected the dephosphorylation of inhibitor-1 by phosphatase-1 , a reaction that was almost completely dependent on MnZ '. Physiological concentrations of Mg2+ (1.0 mM) neither m...

Regulation of Apoptosis Signal-regulating Kinase 1 (ASK1) by PolyamineLevels via Protein Phosphatase5

“…Evidence for proteins directly regulated by polyamines is scarce. In vitro regulation by polyamines has been reported for a PPP phosphatase PP2A (40,55) and for phosphatidylinositol kinases (56); however, polyamine concentrations used in these studies are above the presumed physiological range.…”

“…PP1 is thought to be regulated by polyamine levels in vivo (40 -43). PP2A regulation by polyamines in vitro was reported (40), although in this case effective polyamine concentrations may be above the physiological range. Plant PP7, although not tested with polyamines, was found to be regulated by micromolar concentrations of hexalysine and was suggested to possess a polycation binding site (44).…”

Characterization of a Ribosomal Inhibitory Polypeptide of Protein Phosphatase‐1 from Rat Liver

“…First, RIPP-I was about 5000-fold more potent as an inhibitor of PP-1, (Fig. 4) than was spermine [31,321. Second, the phosphorylase phosphatase activities of PP-1, and PP-2AC are inhibited by similar concentrations of spermine [31], while RIPP-I was a much more specific inhibitor of PP-1, (Fig.…”

Energy metabolism reactions in ruminant muscle: responses to age, nutrition and hormonal status

“…The measured rates of glycogenolysis represent < 1% of the maximal activity of phosphorylase present in muscle and the low expression of potential cycle activity under basal or normal physiological conditions is typical for the majority of substrate cycles. For the G/GIP cycle the simultaneous stimulation of glycogen synthetase and inhibition of phosphorylase by insulin is associated, in part at least, by changes in the activity of protein phosphatases-1 and 2 which regulate the phosphorylation status of the two enzymes (Tung et al, 1985).…”