Protein Dephosphorylation and Protein Phosphorylation

FIG 21.27  Regulation of serine/threonine phosphatases through phosphorylation or intramolecular domain interaction. (a) Phosphorylation of the C- terminal region of PP1c hinders its catalytic activity towards the retinoblastoma protein (Rb). It is suggested that phosphorylation breaches the interaction between the two proteins though other mechanisms could apply. Lack of phosphatase action leads to effective phosphorylation of Rb by CDK2/CyclinE and allows the cell cycle to proceed. (b) PP5 is one of the serine/threonine phosphatases that is not subject to regulation by subunits. Instead, its inactive state is maintained by its TPR domain that wedges into the catalytic site. Glu76 (E76) and the hydrophobic J-helix are highlighted because they stabilize the autoinhibited state. The catalytic His304, adjacent to the Mn2 /Fe2 binding site, is coloured yellow. Proteins that interact with the TPR domain, such as Hsp90, breach the interaction with the J-helix and expose the catalytic site.(1wao130).

Regulation of PPPs

Phosphorylation of the catalytic subunits

The C-terminal regions of the catalytic domains are critical for the communication of regulatory signals to the active site. For instance, phosphorylation by CDK2 of a C-terminal threonine residue in PP1 inhibits the activity in a manner dependent on the point in the cell cycle. This prevents the reversal of CyclinE/CDK2-mediated phosphorylation of the retinoblastoma protein (Rb) and therefore facilitates cell cycle progression towards the S- phase128,129 (Figure 21.27).

Regulation by intramolecular domain interaction

The widely expressed PP5 participates in several stress-activated cellular signalling pathways involving the kinases p38 and JNK (see pages 351 and

463). It also binds to G 12/13, which gives a clue that it may be involved in responses to 7TM receptors.131

The in vitro basal activity of PP5 is extremely low. Moreover, whereas PP1, PP2A, and PP4 exist as dimers or trimers, with the catalytic subunit bound to a regulatory subunit (next section), PP5 is a momomer and regulation is due to the presence of a tetratricopeptide repeat (TPR, 34 residues) domain

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Signal Transduction

present at the N-terminus. The TPR blocks substrate access to the catalytic cleft and this inhibited conformation is stabilized by the J-helical domain at the C-terminus130 (Figure 21.27b). The inhibition can be annulled by G subunits, Hsp90, or arachidonic acid, which reorient the TPR and disrupt the contact with the catalytic domain.

Regulatory subunits of PP1

PP1 is widely distributed and regulates a broad range of cellular functions. These include glycogen metabolism, muscle relaxation (both smooth and skeletal muscle), and cell-cycle progression. The substrate specificities and the regulatory characteristics of muscle and liver type-1 phosphatases are

Table 21.4  Cell regulation by PP1: Examples of the many aspects of signalling controlled by the regulatory subunits of PP1

Continued

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Protein Dephosphorylation and Protein Phosphorylation

Table 21.4  continued

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Signal Transduction

FIG 21.28  Regulatory subunits of PP1. Regulatory subunits engage PP1 in diverse metabolic events and localize the phosphatase to distinct subcellular compartments. Some examples of how regulatory subunits are involved in signalling, complex formation, and the gathering of substrates, kinase and phosphatase, are illustrated. A more comprehensive listing may be found in Table 21.4.

quite distinct, yet the proteins are identical, their differences arising from association with different regulatory subunits.132 About 45 PP1-binding proteins have been identified (Table 21.4). They take part in different processes and they localize the catalytic subunit, PP1c, to different subcellular compartments (Figure 21.28). Although the regulatory subunits are very dissimilar, most, but not all, share a common PP1 binding sequence -K/R-V/I-x- F- (see Figure 21.25b).

In smooth muscle, the regulatory subunit MYPT1 (M110), targets PP1c to myosin and renders it more active against the myosin regulatory light chain. The main outcome of the interaction of MYPT1 with PP1c is the formation of an extended acidic groove, well adapted to accommodate the basic N-terminal sequence of myosin and making it less attractive for other substrates.133 This is illustrated in Figure 21.29a.

Besides defining substrate specificity, regulatory subunits also allow the activity of PP1c to be modulated by phosphorylation or by second messengers. For instance, Rho-mediated regulation of smooth muscle

contraction and the formation of focal adhesion contacts are exerted in part by the Rho-regulated kinase ROCK. This phosphorylates the MYPT1 subunit, so suppressing PP1 activity towards the myosin regulatory light chain.

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