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In the present study, we identified protein phosphatases dephosphorylating centrins previously phosphorylated by protein kinase CK2. The following phosphatases known to be present in the retina were tested: PP1, PP2A, PP2B, PP2C, PP5, and alkaline phosphatase. PP2C α and β were capable of dephosphorylating P-Thr138-centrin1 most efficiently. PP2Cδ was inactive and the other retinal phosphatases also had much less or no effect. Similar results were observed for centrins 2 and 4. Centrin3 was not a substrate for CK2. The results suggest PP2C α and β to play a significant role in regulating the phosphorylation status of centrins in vivo.
In the highly specialized vertebrate photoreceptor cells, centrins are components of the ciliary apparatus localized in the connecting cilium and their basal bodies [1–3]. In fully differentiated photoreceptor cells, CK2 phosphorylates centrin1 and 2 during dark adaptation. Since the phosphorylation of the ciliary centrins drastically reduces the binding to the G-protein transducin, it is suggested that the light-dependent translocation of transducin through the cilium is further regulated by CK2 phosphorylation and by the phosphatase involved.
The present study was designed to identify protein phosphatases that serve as counterparts for the CK2-mediated light-dependent phosphorylation of centrins in mammalian photoreceptor cells.
GST-centrins (0.2μg) or GST-BAD (0.6μg) were incubated in 30mM Tris-HCl, pH 7.5, 5mM MgCl2, 5mM β-glycerophosphate, 0.2μg CK2, 0.06% 2-mercaptoethanol, 1mM EGTA, and 100μM ATP including 1μCi [γ − 32P]ATP in a volume of 10μL for 15 minutes at 37°C. Then unincorporated ATP was removed by centri-SEP spin colums.
Phosphorylated proteins were incubated with 0.16μg PP1 or 0.05μg PP2A or 1.3μg PP2B or 0.08–0.8μg PP2Cα or 0.08–1.5μg PP2Cβ or 0.08–0.8μg PP2Cδ or 0.8μg PP5 or 1.5μg alkaline phosphatase in a total volume of 15μL, respectively. Incubations contained a 10μL aliquot of the completed phosphorylation reaction plus 5μL 50mM Tris-HCl, pH 7.5, 1% glycerol, 0.1% 2-mercaptoethanol, and an additional 5mM MnCl2 for PP1, PP2A, and PP2Cδ; or 1mM MgCl2, 0.1mM CaCl2, and 2μg calmodulin for PP2B; or 1mM MgCl2 for PP2C α and β; or 100μM oleic acid for PP5. Alkaline phosphatase assays contained 50mM Tris-HCl, pH 7.9 and 1mM MgCl2. Reactions were stopped after 30 minutes at 37°C by adding 5μL sample buffer (130mM Tris-HCl, pH 6.8, 10% SDS, 10% 2-mercaptoethanol, 20% glycerol, 0.06% bromphenol blue).
Purified recombinant centrin1 could be phosphorylated in vitro by CK2 using ATP as phosphate source within a few minutes only (Figure 1(a)). Phosphorylation of centrin1 by CK2 was not detectable in the presence of 100μM of the CK2-inhibitor TBB (Figure 1(b)), left). Guanine nucleotides are playing a uniquely important role in the retina and for vision . Indeed, phosphorylation of centrin1 by CK2 worked equally well using GTP as phosphate source instead of ATP (Figure 1(c)).
Thr138 of centrin1 is conserved in centrin2 (Thr137) and centrin4 (Thr134) whereas centrin3 (Ser135) carries a serine residue instead (Figure 1(f)). As expected from the amino acid sequence identity, centrins 2 and 4 also could be phosphorylated by CK2 (Figure 1(d)). A variety of proteins are phosphorylated by CK2 at serine residues (for review see ). Centrin3, however, was not a substrate of CK2 (Figure 1(d)). Coomassie staining was used in parallel to verify equal protein loading (Figure 1(e)).
Phosphatases acting on P-centrin1 included PP1, PP2A, PP2B, PP2Cβ, and PP5. Unspecific alkaline phosphatase was also tested. The CK2-inhibitor TBB used to prevent ongoing phosphorylation upon incubation with the phosphatases had no effect on the phosphatase activities as exemplified for PP2Cβ (Figure 1(b), right).
Among the 6 phosphatases tested here PP2Cβ was most efficiently dephosphorylating P-centrin1 (Figure 2(a)). All the other phosphatases tested had no or much less effect (Figure 2(a)). This unexpected selectivity prompted us to run the dephosporylation of P-BAD as an extra control. For that purpose BAD was phosphorylated at Thr117 by CK2 . Dephosporylation of P-BAD was run in parallel and identical to the experiments dealing with the putative dephosphorylation of P-centrin1. In analogy to what is known for the majority of phosphorylation sites in any protein, our in vitro studies revealed that P-Thr117-BAD more or less could be hydrolyzed by all the phosphatases tested (Figure 2(b)). This was in sharp contrast to the results obtained with phosphatases acting on P-centrin1 (Figure 2(a) versus 2(b)). This unexpected result—strongest dephosphorylation of P-centrin1 by PP2Cβ (Figure 2(a))—was also observed for P-centrins2 and 4 (data not shown).
An increasing amount of PP2Cβ protein resulted in enhanced dephosphorylation (Figure 3(a)). PP2C enzymes are characterized by their requirement for Mg2+- or Mn2+-cations for activity . In line with that, dephosphorylation of P-centrin1 by PP2Cβ increased upon addition of Mg2+-ions (Figure 3(b)). Increasing the Ca2+-ion concentration reduced dephosphorylation of P-centrin1 by PP2Cβ (Figure 3(c)). Unsaturated long-chain fatty acids are inhibiting PP2C activity from plants  but activate PP2Cα and PP2Cβ in vertebrates . Oleic acid (18 : 1) was capable of stimulating dephosphorylation of P-centrin1 by PP2Cβ (Figure 3(d)).
Phosphorylation of centrins by CK2 occurs during dark adaptation in photoreceptor cells of the mammalian retina. It reciprocally regulates the Ca2+-mediated binding of centrins to the βγ-subunit of the visual heterotrimeric G-protein transducin [1, 9, 10]. If CK2 is constantly active in photoreceptor cilia, as seen in most systems studied so far, the identity and regulation of a phosphatase responsible for dephosphorylation of CK2-mediated centrin phosphorylation might be crucial for the biological effect of centrins.
Accordingly, in the present study, we addressed the question which phosphatase is capable of dephosphorylating P-Thr138-centrin1. All the most abundant retinal phosphatases were tested, that is, PP1, PP2A, PP2B, PP2C α and β, PP5, and alkaline phosphatase [11–14]. Our results were most striking: PP2C α and β most efficiently hydrolyzed P-centrin1; all other phosphatases tested had no or much less effect. This unexpected finding was verified using P-Thr117-BAD, phosphorylated by CK2, for control . As expected, P-BAD was dephosphorylated by all those phosphatases which is in sharp contrast to the dephosphorylation of P-centrin1 by PP2C α and β.
Many proteins are phosphorylated at several distinct sites. Knowledge on the reversible phosphorylation of centrins currently comprises PKA at Ser167 [15–17], PKC , Cdc2 , and CK2 . This report is the first focusing on phosphatases acting on P-centrins. Because of the unexpected potency of PP2C α and β to dephosphorylate CK2-mediated P-centrin1, we briefly checked whether PP2C α and β might also dephosphorylate P-centrin1 after phosphorylation by PKA. This was not the case (data not shown). Therefore, we conclude that if there is crosstalk and hierarchy among the two phosphorylation sites identified in centrin proteins, PP2C α and β are playing a most decisive role. Overall, dephosphorylation of P-centrins by PP2C α and β should increase the affinity of centrins to Gtβγ and finally reduce transport of the G-protein transducin through the connecting cilium.