SH2 Domains: Folding, Binding and Therapeutical Approaches

Abstract

SH2 (Src Homology 2) domains are among the best characterized and most studied protein-protein interaction (PPIs) modules able to bind and recognize sequences presenting a phosphorylated tyrosine. This post-translational modification is a key regulator of a plethora of physiological and molecular pathways in the eukaryotic cell, so SH2 domains possess a fundamental role in cell signaling. Consequently, several pathologies arise from the dysregulation of such SH2-domains mediated PPIs. In this review, we recapitulate the current knowledge about the structural, folding stability, and binding properties of SH2 domains and their roles in molecular pathways and pathogenesis. Moreover, we focus attention on the different strategies employed to modulate/inhibit SH2 domains binding. Altogether, the information gathered points to evidence that pharmacological interest in SH2 domains is highly strategic to developing new therapeutics. Moreover, a deeper understanding of the molecular determinants of the thermodynamic stability as well as of the binding properties of SH2 domains appears to be fundamental in order to improve the possibility of preventing their dysregulated interactions.

Keywords: Src Homology 2; phosphotyrosine; protein-protein interactions.

Figure 1

(Left) structure of Shp2 protein composed of two contiguous SH2 domains (N- and C-SH2, in green and purple, respectively) and a PTP catalytic domain. PDBcode: 2shp. (Right) Folding characterization of N-SH2 (reproduced with permission from [19]) and C-SH2 (reproduced under the terms of the Creative Commons Attribution License from [20]) domains of Shp2. The ϕ value analysis carried out using the isolated domains allows for defining the native-like interactions occurring in the transition state(s) of the folding reaction. Data obtained were then used to obtain a Φ vs. Φ plots (in the center) of early (top left panel) and late (top right panel) events of the folding reaction of N-SH2 versus C-SH2 domain (see references and text for details).

Figure 2

(A) Scheme of a canonical binding reaction between an SH2 domain (in green) and a Tyrosine-phosphorylated peptide (in violet). The peptide is numbered by counting pTyr as 0. In detail, we can distinguish two structural binding elements in the SH2 domain; pTyr pocket—composed by helix αA (in cyano), beta-strands βB, βC, and βD (in light green), and the BC-loop—and a specificity pocket—composed by helix αB (dark green), beta-strands βD, βE, βF and βG (light green) BG loop and the EF loop. Arg³² in the pTyr pocket (green circle) is a highly conserved residue driving the interaction in all SH2 domains. Arg/Lys on αA or βD (white circles) can assist the reaction. (B) Examples of interactions between SH2 domains and pTyr-peptides mediated by Arg32 and an Arg residue on αA in case of Src-like class of binding (left, PDBcode:1sps), and an Arg residue on βD in case of Sap-like class of binding (right, PDBcode:1d4w). Structural details in the squares. The conserved binding motif is highlighted in blue. The motif corresponds to FLVR and YLVR in reported examples of Src-like (canonical) and Sap-like (non-canonical) classes of binding, respectively.

Figure 3

(A) A representative example of an SH2 domain in complex with a peptide mimicking a physiological ligand (PDBcode 1TCE). In detail, the SH2 domain of Shc (in light cyan) can interact with a tyrosine-phosphorylated peptide from the T cell receptor (in yellow) through a highly conserved sequence FLVRES (highlighted in blue). Arg32 (blue stick) is crucial for the interaction with the pY (yellow stick) harbored by the ligand. (B) Schematic representation of different SH2-domain-containing proteins exerting different molecular functions. In brackets, all protein families split according to the function (see Table 1 for references).

Figure 4

Oncogenic signaling pathways activated by interaction between the SH2 domain of different proteins (Grb2, p85PI3K, Stat). Phosphorylated sites are highlighted.