DOC_PP1_SILK_1

The ubiquitous serine ⁄ threonine protein phosphatases of type 1 (PP1) and type 2A (PP2A) belong to the phosphoprotein phosphatase (PPP) superfamily (Peti,2013). Together, they are responsible for more than 90 % of the protein phosphatase activity in eukaryotic cells. PP1 regulates diverse cellular processes such as cell cycle progression, apoptosis, protein synthesis, muscle contraction, glycogen metabolism, transcription and neuronal signalling (Hurley,2007; Aggen,2000). Three different genes in the mammalian genome encode four distinct catalytic subunits PP1α, PP1β⁄δ and the splice variants PP1γ1 and PP1γ2. These isoforms show a sequence identity of about 90% and most of the amino acid differences are found in the N- and C-terminal extremities. While Saccharomyces cerevisiae with only one PP1 gene (Glc7) is an exception, many Bacteria759 have multiple PP1 genes: eight are found in Arabidopsis thaliana, four in Drosophila melanogaster and 30 are predicted in Caenorhabditis elegans (Moorhead,2007, Wakula,2003).
With more than 200 target proteins the catalytic subunit (PP1c) lacks substrate specificity (Bollen,2010). It is recruited by the regulatory subunits to dephosphorylate phosphothreonine or phosphoserine residues in the target substrates. Using one or more of these regulators, PP1c is converted into hundreds of very specific holoenzymes. The catalytic site of PP1c is located at the intersection of three substrate-binding grooves: the acidic, the hydrophobic and the C-terminal binding groove (Terrak,2004). It is surrounded by acidic residues and contains two metal ions. In mammalian tissues these ions are Fe2+ and Zn2+, whereas in bacteria Mn2+ is incorporated (Bollen,2010).
PP1-interacting proteins (PIPs) function as targeting subunits, inhibitors and substrates. More than half of all PIPs inhibit PP1 when glycogenphosphorylase is used as a substrate. The IPP-1 (Protein phosphatase inhibitor 1, Q13522) protein and its homolog DARPP-32 (Dopamine- and cAMP-regulated phosphoprotein, Q9UD71) bind to and block the PP1 catalytic site after being phosphorylated on a specific threonine residue, thus acting as pseudosubstrates, while IPP-2 (Protein phosphatase inhibitor 2, P41236) does not require prior phosphorylation for the formation of a stable inactive complex with PP1c (Huang,1999; Connor,2000).
PIPs are predicted to contain regions that are intrinsically disordered, an attribute that favours their binding to a large surface area of PP1 via multiple docking motifs. Most of the known PP1-docking motifs are short, highly conserved and their flanking regions contain less conserved segments (Hendrickx,2009). They play an essential role in the regulation of PP1 activity and substrate specificity. Generally, PIPs contain multiple distinct PP1-docking motifs, which likely mediate cooperative binding to PP1. The RVXF motif (DOC_PP1_RVXF_1) is present in more than 90% of all known PP1 regulators. It functions primarily as an anchoring motif, as it does not influence the structure of PP1. The RVXF binding site is ~20 Å away from the PP1c active site. The hydrophobic groove of PP1c interacts with the RVXF motif in regulatory proteins, binding the peptide backbone by beta-strand augmentation (Peti,2013; Moorhead,2007). The SILK motif (DOC_PP1_SILK_1) is highly conserved among Opisthokonta and Amoebozoa and occurs in IPP-2 and several other vertebrate PP1 interactors. It binds in the hydrophobic groove at the back side of PP1 and is located N-terminal to the RVXF motif in proteins where these two motifs co-occur (Dancheck,2011; Hendrickx,2009). Similar to the RVXF motif, the SILK motif acts as an anchoring site and does not change the conformation of PP1. The myosin phosphatase N-terminal element or MyPhoNE motif contributes to substrate selection and is present in MYPT1 (Myosin phosphatase target subunit 1, O14974) and a few other PIPs. It is also located N-terminal to the RVXF sequence when both are present, lying within a five-turn α-helix that faces hydrophobic residues in a shallow hydrophobic cleft on PP1 (Bollen,2010).