Protein phosphatase 1 structure: How MYPT1 adds specificity

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Substrate specificity of the Ser/Thr protein phosphastase 1 (PP1) is provided by regulatory subunits. The crystal structure of PP1 and myosin phosphatase targeting subunit (MYPT1) shows how the catalytic cleft of PP1 is reshaped to increase PP1's catalytic activity and specificity for myosin.

The reciprocal regulation of a plethora of substrates by Serine/threonine (Ser/Thr) protein kinases and phosphatases is at the heart of signal transduction. This well-tuned balance of transient phosphorylation occurs even though Ser-Thr kinases significantly outnumber phosphatases. The ubiquitous protein phosphatase 1 (PP1) is an effective catalyst but lacks inherent substrate specificity, which is brought about by interaction with a spectrum of regulatory subunits that only share a canonical PP1-binding sequence known as the RVxF motif. Terrak et al. now show that structural elements of the myosin phosphatase targeting subunit (MYPT1) reshape the catalytic cleft of PP1, thereby increasing PP1's catalytic activity and specificity for myosin.

Smooth muscle relaxation occurs through the dephosphorylation of myosin. Myosin phosphatase is made up of a trimeric holoenzyme consisting of MYPT1, PP1 and M20, a protein of unknown function. Binding of PP1 to the N-terminal region of MYPT1 produces a 15-fold increase in catalytic activity and a 10-fold higher affinity for phopshorylated myosin. The authors report the 2.7-Â-resolution structure of the PP1 delta -MYPT1 complex.

The overall structure of PP1 consists of two tightly linked domains, an N-terminal alpha / beta domain, and a C-terminal beta domain that also contains three a-helices. The beta -strands converge to a beta -sandwich at the inter-domain interface. At this interface the catalytic site resides within a large Y-shaped cleft formed by three converging grooves (the hydrophobic, acidic and C-terminal grooves). Upon binding, the 34 N-terminal amino acids of MYPT1 proximal to the RVxF motif, form a long arm that wraps around PP1 to reach the base of the Y-shaped catalytic cleft.

Amino acids 39-291 of MYPT1 fold into two groups of four ankyrin repeats separated by a hinge at Glu 172. MYPT1 interacts with PP1 via repeats 1, 5, 6, and 7, with repeat 1 interacting with the RVxF-binding pocket. Not only do the ankyrin repeats play a role in binding, but they also play a key role in the modulation of PP1 catalytic activity: beta -hairpin loops from the two groups of ankyrin repeats face each other forming a clamp-like structure that closes around the C-terminus of PP1, and this gives rise to a large acidic cleft which is positioned so as to extend the catalytic cleft of PP1. This extended catalytic cleft is specifically adapted for the myosin substrate and is less compatible with other substrates. In this way, the regulatory subunit adapts the catalytic cleft of PP1 to perform a specific function. These features of the PP1-MYPT1 complex have general implications for how other PP1 regulatory subunits work.

Brenda Riley, Assistant Editor
Signaling Gateway

References

Terrak, Mohammed, Kerff, Frederic, Langsetmo, Knut, Tao, Terence, & Dominguez, Roberto Structural basis of protein phosphatase 1 regulation. Nature, 429, 780–784 (17 June 2004); 10.1038/nature02582 | Article |