Evolutionary crossroads of cell signaling: PP1 and PP2A substrate sites in intrinsically disordered regions.

Maja Köhn,Bernhard Hoermann

doi:10.1042/bst20200175

Abstract

Phosphorylation of the hydroxyl group of the amino acids serine and threonine is among the most prevalent post-translational modifications in mammalian cells. Phospho-serine (pSer) and -threonine (pThr) represent a central cornerstone in the cell's toolbox for adaptation to signal input. The true power for the fast modulation of the regulatory pSer/pThr sites arises from the timely attachment, binding and removal of the phosphate. The phosphorylation of serine and threonine by kinases and the binding of pSer/pThr by phosphorylation-dependent scaffold proteins is largely determined by the sequence motif surrounding the phosphorylation site (p-site). The removal of the phosphate is regulated by pSer/pThr-specific phosphatases with the two most prominent ones being PP1 and PP2A. For this family, recent advances brought forward a more complex mechanism for p-site selection. The interaction of regulatory proteins with the substrate protein constitutes a first layer for substrate recognition, but also interactions of the catalytic subunit with the amino acids in close proximity to pSer/pThr contribute to p-site selection. Here, we review the current pieces of evidence for this multi-layered, complex mechanism and hypothesize that, depending on the degree of higher structure surrounding the substrate site, recognition is more strongly influenced by regulatory subunits away from the active site for structured substrate regions, whereas the motif context is of strong relevance with p-sites in disordered regions. The latter makes these amino acid sequences crossroads for signaling and motif strength between kinases, pSer/pThr-binding proteins and phosphatases.

Highlights

To react and adapt to changes, cells have developed more than 400 different post-translational modifications (PTMs), which act as dynamic modulators upon attachment to amino acid (AA) side chains in order to modify protein stability, activity, structure, localization and binding [1]
We find in our own data that p-sites sensitive to PP1 catalytic core protein (PP1c) and PP2Ac differ, depending on their structural characteristics: For PP1c, we find that the enrichment of Arg in −3 is only important in disordered regions, whereas position −1 plays a more important role in ordered domains (Figure 2A,B)
Given the recent insights presented in this review, we anticipate that disentangling the phosphoproteome in terms of sequence context and structural features will strongly help to elucidate the regulatory relevance of p-sites and the mechanism of p-site recognition by PP1 and PP2A in the upcoming years, and will support completing the understanding of the interplay between kinases, phosphatases, and interacting proteins with phosphoprotein p-sites

Summary

Introduction

To react and adapt to changes, cells have developed more than 400 different post-translational modifications (PTMs), which act as dynamic modulators upon attachment to amino acid (AA) side chains in order to modify protein stability, activity, structure, localization and binding [1]. Advances in phosphoproteomics have led to a drastic increase in phosphorylation site ( p-site) annotations over the last 15 years: Whereas at the beginning of the millennium the majority of the known phosphoproteome still consisted of p-sites identified and characterized by directed lowthroughput (LTP) efforts aiming to study the role of single p-sites [8], this picture changed concomitant with the enormous speed of technological progress in subsequent years This development is well reflected in a recent study by Beltrao and collaborators, combining knowledge of more than 6000 phosphoproteome experiments executed across human cell types. These numbers represent the to date most complete view on the human phosphoproteome [3]

Objectives

Findings

Conclusion