Preparation and characterization of recombinant protein phosphatase 1

The dominant protein phosphatase PP1c isoform in smooth muscle cells, PP1cβ, is essential for smooth muscle contraction

Contractile force development of smooth muscle is controlled by balanced kinase and phosphatase activities toward the myosin regulatory light chain (RLC). Numerous biochemical and pharmacological studies have investigated the specificity and regulatory activity of smooth muscle myosin light-chain phosphatase (MLCP) bound to myosin filaments and comprised of the regulatory myosin phosphatase target subunit 1 (MYPT1) and catalytic protein phosphatase 1cβ (PP1cβ) subunits. Recent physiological and biochemical evidence obtained with smooth muscle tissues from a conditional MYPT1 knockout suggests that a soluble, MYPT1-unbound form of PP1cβ may additionally contribute to myosin RLC dephosphorylation and relaxation of smooth muscle. Using a combination of isoelectric focusing and isoform-specific immunoblotting, we found here that more than 90% of the total PP1c in mouse smooth muscles is the β isoform. Moreover, conditional knockout of PP1cα or PP1cγ in adult smooth muscles did not result in an apparent phenotype in mice up to 6 months of age and did not affect smooth muscle contractions ex vivo In contrast, smooth muscle-specific conditional PP1cβ knockout decreased contractile force development in bladder, ileal, and aortic tissues and reduced mouse survival. Bladder smooth muscle tissue from WT mice was selectively permeabilized to remove soluble PP1cβ to measure contributions of total (α-toxin treatment) and myosin-bound (Triton X-100 treatment) phosphatase activities toward phosphorylated RLC in myofilaments. Triton X-100 reduced PP1cβ content by 60% and the rate of RLC dephosphorylation by 2-fold. These results are consistent with the selective dephosphorylation of RLC by both MYPT1-bound and -unbound PP1cβ forms in smooth muscle.

Characterization of the interactions between inhibitor-1 and recombinant PP1 by NMR spectroscopy

Inhibitor-1 is converted into a potent inhibitor of native protein phosphatase-1 (PP1) when Thr35 is phosphorylated by cAMP-dependent protein kinase (PKA). However, PKA-phosphorylated form of inhibitor-1 displayed a weak activity in inhibition of recombinant PP1. The mechanism for the impaired activity of PKA-phosphorylated inhibitor-1 toward inhibition of recombinant PP1 remained elusive. By using NMR spectroscopy in combination with site-directed mutagenesis and inhibitory assay, we found that the interaction between recombinant PP1 and the consensus PP1-binding motif of PKA-thiophosphorylated form of inhibitor-1 was unexpectedly weak. Unlike binding to native PP1, the subdomains 1 (residues around and including the phosphorylated Thr35) and 2 (the consensus PP1-binding motif) of PKA-thiophosphorylated form of inhibitor-1 do not exhibit a synergistic effect in inhibition of recombinant PP1. This finding implied that a slight structural discrepancy exists between native and recombinant PP1, resulting in PKA-thiophosphorylated form of inhibitor-1 displaying a different affinity to native and recombinant enzyme.

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

In vitro reconstitution of a CaMKII memory switch by an NMDA receptor-derived peptide

Ca(2+)/Calmodulin-dependent protein kinase II (CaMKII) has been shown to play a major role in establishing memories through complex molecular interactions including phosphorylation of multiple synaptic targets. However, it is still controversial whether CaMKII itself serves as a molecular memory because of a lack of direct evidence. Here, we show that a single holoenzyme of CaMKII per se serves as an erasable molecular memory switch. We reconstituted Ca(2+)/Calmodulin-dependent CaMKII autophosphorylation in the presence of protein phosphatase 1 in vitro, and found that CaMKII phosphorylation shows a switch-like response with history dependence (hysteresis) only in the presence of an N-methyl-D-aspartate receptor-derived peptide. This hysteresis is Ca(2+) and protein phosphatase 1 concentration-dependent, indicating that the CaMKII memory switch is not simply caused by an N-methyl-D-aspartate receptor-derived peptide lock of CaMKII in an active conformation. Mutation of a phosphorylation site of the peptide shifted the Ca(2+) range of hysteresis. These functions may be crucial for induction and maintenance of long-term synaptic plasticity at hippocampal synapses.

Reconstituted human myosin light chain phosphatase reveals distinct roles of two inhibitory phosphorylation sites of the regulatory subunit, MYPT1

The myosin light chain phosphatase (MLCP) is a cytoskeleton-associated protein phosphatase-1 (PP1) holoenzyme and a RhoA/ROCK effector, regulating cytoskeletal reorganization. ROCK-induced phosphorylation of the MLCP regulatory subunit (MYPT1) at two sites, Thr696 and Thr853, suppresses the activity, although little is known about the difference in the role. Here, we developed a new method for the preparation of the recombinant human MLCP complex and determined the molecular and cellular basis of inhibitory phosphorylation. The recombinant MLCP partially purified from mammalian cell lysates retained characteristics of the native enzyme, such that it was fully active without Mn²⁺ and sensitive to PP1 inhibitor compounds. Selective thio-phosphorylation of MYPT1 at Thr696 with ROCK inhibited the MLCP activity 30%, whereas the Thr853 thio-phosphorylation did not alter the phosphatase activity. Interference with the docking of phospho-Thr696 at the active site weakened the inhibition, suggesting selective autoinhibition induced by phospho-Thr696. Both Thr696 and Thr853 sites underwent autodephosphorylation. Compared with that of Thr853, phosphorylation of Thr696 was more stable, and it facilitated Thr853 phosphorylation. Endogenous MYPT1 at Thr696 was spontaneously phosphorylated in quiescent human leiomyosarcoma cells. Serum stimulation of the cells resulted in dissociation of MYPT1 from myosin and PP1C in parallel with an increase in the level of Thr853 phosphorylation. The C-terminal domain of human MYPT1(495–1030) was responsible for the binding to the N-terminal portion of myosin light meromyosin. The spontaneous phosphorylation at Thr696 may adjust the basal activity of cellular MLCP and affect the temporal phosphorylation at Thr853 that is synchronized with myosin targeting.

The nuclear envelope protein, LAP1B, is a novel protein phosphatase 1 substrate

Protein phosphatase 1 (PP1) binding proteins are quintessential regulators, determining substrate specificity and defining subcellular localization and activity of the latter. Here, we describe a novel PP1 binding protein, the nuclear membrane protein lamina associated polypeptide 1B (LAP1B), which interacts with the DYT1 dystonia protein torsinA. The PP1 binding domain in LAP1B was here identified as the REVRF motif at amino acids 55-59. The LAP1B:PP1 complex can be immunoprecipitated from cells in culture and rat cortex and the complex was further validated by yeast co-transformations and blot overlay assays. PP1, which is enriched in the nucleus, binds to the N-terminal nuclear domain of LAP1B, as shown by immunocolocalization and domain specific binding studies. PP1 dephosphorylates LAP1B, confirming the physiological relevance of this interaction. These findings place PP1 at a key position to participate in the pathogenesis of DYT1 dystonia and related nuclear envelope-based diseases.

Identification and characterization of two distinct PPP1R2 isoforms in human spermatozoa

Background

Protein Ser/Thr Phosphatase PPP1CC2 is an alternatively spliced isoform of PPP1C that is highly enriched in testis and selectively expressed in sperm. Addition of the phosphatase inhibitor toxins okadaic acid or calyculin A to caput and caudal sperm triggers and stimulates motility, respectively. Thus, the endogenous mechanisms of phosphatase inhibition are fundamental for controlling sperm function and should be characterized. Preliminary results have shown a protein phosphatase inhibitor activity resembling PPP1R2 in bovine and primate spermatozoa.

Results

Here we show conclusively, for the first time, that PPP1R2 is present in sperm. In addition, we have also identified a novel protein, PPP1R2P3. The latter was previously thought to be an intron-less pseudogene. We show that the protein corresponding to the pseudogene is expressed. It has PPP1 inhibitory potency similar to PPP1R2. The potential phosphosites in PPP1R2 are substituted by non-phosphorylable residues, T73P and S87R, in PPP1R2P3. We also confirm that PPP1R2/PPP1R2P3 are phosphorylated at Ser121 and Ser122, and report a novel phosphorylation site, Ser127. Subfractionation of sperm structures show that PPP1CC2, PPP1R2/PPP1R2P3 are located in the head and tail structures.

Conclusions

The conclusive identification and localization of sperm PPP1R2 and PPP1R2P3 lays the basis for future studies on their roles in acrosome reaction, sperm motility and hyperactivation. An intriguing possibility is that a switch in PPP1CC2 inhibitory subunits could be the trigger for sperm motility in the epididymis and/or sperm hyperactivation in the female reproductive tract.

Structural basis for protein phosphatase 1 regulation and specificity

The ubiquitous serine/threonine protein phosphatase 1 (PP1) regulates diverse, essential cellular processes such as cell cycle progression, protein synthesis, muscle contraction, carbohydrate metabolism, transcription and neuronal signaling. However, the free catalytic subunit of PP1, while an effective enzyme, lacks substrate specificity. Instead, it depends on a diverse set of regulatory proteins (≥ 200) to confer specificity towards distinct substrates. Here, we discuss recent advances in structural studies of PP1 holoenzyme complexes and summarize the new insights these studies have provided into the molecular basis of PP1 regulation and specificity.

SARP, a new alternatively spliced protein phosphatase 1 and DNA interacting protein

PP1 (protein phosphatase 1) is a ubiquitously expressed serine/threonine-specific protein phosphatase whose activity towards different substrates appears to be mediated via binding to specific proteins that play critical regulatory and targeting roles. In the present paper we report the cloning and characterization of a new protein, termed SARP (several ankyrin repeat protein), which is shown to interact with all isoforms of PP1 by a variety of techniques. A region encompassing a consensus PP1-binding motif in SARP (K354VHF357) modulates endogenous SARP-PP1 activity in mammalian cells. This SARP-PP1 interaction motif lies partially within the first ankyrin repeat in contrast with other proteins [53BP2 (p53 binding protein 2), MYPT1/M(110)/MBS (myosin binding protein of PP1) and TIMAP (transforming growth factor beta inhibited, membrane-associated protein)], where a PP1-binding motif precedes the ankyrin repeats. Alternative mRNA splicing produces several isoforms of SARP from a single human gene at locus 11q14. SARP1 and/or SARP2 (92-95 kDa) are ubiquitously expressed in all tissues with high levels in testis and sperm, where they are shown to interact with both PP1gamma1 and PP1gamma2. SARP3 (65 kDa) is most abundant in brain where SARP isoforms interact with both PP1alpha and PP1gamma1. SARP is highly abundant in the nucleus of mammalian cells, consistent with the putative nuclear localization signal at the N-terminus. The presence of a leucine zipper near the C-terminus of SARP1 and SARP2, and the binding of mammalian DNA to SARP2, suggests that SARP1 and SARP2 may be transcription factors or DNA-associated proteins that modulate gene expression.

Protein phosphatase 1 positively regulates stomatal opening in response to blue light in Vicia faba

Phototropins, plant blue light receptors, mediate stomatal opening through the activation of the plasma membrane H(+)-ATPase by unknown mechanisms. Here we report that type 1 protein phosphatase (PP1) positively regulates the blue light signaling between phototropins and the H(+)-ATPase in guard cells of Vicia faba. We cloned the four catalytic subunits of PP1 (PP1c) from guard cells and determined the expression of the isoforms in various tissues. Transformation of Vicia guard cells with PP1c isoforms that had lost enzymatic activity by one amino acid mutation, or with human inhibitor-2, a specific inhibitor protein of PP1c, suppressed blue light-induced stomatal opening. Addition of fusicoccin, an activator of the plasma membrane H(+)-ATPase, to these transformed guard cells induced normal stomatal opening, suggesting that the transformations did not affect the basic mechanisms for stomatal opening. Tautomycin, an inhibitor of PP1, inhibited blue light-induced H(+) pumping, phosphorylation of the plasma membrane H(+)-ATPase in guard cell protoplasts, and stomatal opening. However, tautomycin did not inhibit the blue light-dependent phosphorylation of phototropins. We conclude that PP1 functions downstream of phototropins and upstream of the H(+)-ATPase in the blue light signaling pathway of guard cells.

Phospho-Pivot Modeling Predicts Specific Interactions of Protein Phosphatase-1 with a Phospho-Inhibitor Protein CPI-17

Phospho-amino acids in proteins are directly associated with phospho-receptor proteins, including protein phosphatases. Here we produced and tested a scheme for docking together interacting phospho-proteins whose monomeric 3D structures were known. The phosphate of calyculin A, an inhibitor for protein phosphatase-1 and 2A (PP1 and PP2A), or phospho-CPI-17, a PP1-specific inhibitor protein, was docked at the active site of PP1. First, a library of 186,624 virtual complexes was generated in silico, by pivoting the phospho-ligand at the phosphorus atom by step every 5 degrees on three rotational axes. These models were then graded for probability according to atomic proximity between two molecules. The predicted structure of PP1 x calyculin A complex fitted to the crystal structure with r.m.s.d. of 0.23 A, providing a validate test of the modeling method. Modeling of PP1 x phospho-CPI-17 complex yielded one converged structure. The segment of CPI-17 around phospho-Thr38 is predicted to fit in the active site of PP1. Positive charges at Arg33/36 of CPI-17 are in close proximity to Glu274 of PP1, where the sequence is unique among Ser/Thr phosphatases. Single mutations of these residues in PP1 reduced the affinity against phospho-CPI-17. Thus, the interface of the PP1 x CPI-17 complex predicted by the phospho-pivot modeling accounts for the specificity of CPI-17 against PP1.