SH2 and SH3 domains

Src tyrosine kinases comprise SH2 and SH3 domains in addition to a tyrosine kinase... 

C-terminal lobes of the tyrosine kinase are similar to those of cyclin-depen-dent kinase described in Chapter 6 (see Figure 6.16a), while the SH2 and SH3 domains of Src and Hck have structures very similar to those of the isolated domains (see Figures 13.26 and 13.28a). 

Koch CA, Anderson D, Moran MF, Ellis C, Pawson T. SH2 and SH3 domains elements that control interactions of cytoplasmic signaling proteins. Science 1991 252 668-674. 

Many non-receptor tyrosine kinases have been identified as products of retrovirally encoded oncogenes. Non-receptor tyrosine kinases can be divided into two groups transmembrane and cytosolic families. The c-src tyrosine kinase is the prototype of the cytosolic tyrosine kinases. Regions within these non-receptor tyrosine kinases share homology with the Src kinase, known as Src homology 2 and 3 (SH2 and SH3) domains, and mediate protein-protein interactions between the receptor tyrosine kinases and the intracellular targets (reviewed in Cantley et al., 1991 Pawson and Gish, 1992 Mayer and Baltimore, 1993). 

FIGURE 8.13 Nonreceptor PTKs. These protein kinases form a large family, and most of them contain SH2 and SH3 domains. Several were originally discovered as transforming genes of a viral genome, hence names such as src or abl, derived from Rous sarcoma virus or Abelson murine leukemia virus, respectively. (Adapted from Hunter, T., Biochem. Soc. Trans., 24(2), 307-327, 1996.)... 

Despite the similar functions of each isozyme, only two regions of amino acid homology exist (X and Y), one of 150 and a second of 120 amino acid residues, which are 54% and 42% identical among the isozymes but are differentially localized within each enzyme (Fig. 20-3). The X and Y domains form the catalytic core of the enzyme. A characteristic of the (3 and 8 isoforms is that relatively few amino acids (40-110) separate the X and Y entities, whereas a much larger separation is observed for the PLCy isoform (approx. 400). In addition, in PLCy, the region between X and Y contains amino acid sequences that are found in nonreceptor tyrosine kinases (SH2 and SH3 domains). All four isoforms possess pleckstrin homology (PH) domains. The latter are considered to enable the enzyme to become tethered to the plasmalemma via an interaction with PI(4,5)P2. In addition, all PLC isoforms possess an E-F hand domain, which is located between PH and X domains, and a C2 domain, which is located close to the Y domain. 

SH2 and SH3 domains refer to src homology 2 and 3, a non-receptor tyrosine kinase where these domains were identified originally. They bind to phos-phorylated tyrosine and pro line rich epitopes, respectively, with specificities each in respect to the neighboring amino acids. Grb2 is an adapter molecule par excellence having both SH2 and SH3 domains (see also the respective article in this volume). 

The functions of transmembrane receptors can be modified using adaptor motecules. Sometimes the adaptors bring in substrates to the receptor s enzyme activity, but other times they can bring in the activity itsetf. The best known adaptors have protein structurai domains called SH2 or SH3 domains. These adaptors couple various functions to receptor tyrosine kinases (Fig. 9-4). SH2 and SH3 stands for Src homotogy region 2 and 3 because they were discovered first in the oncogene, Src (see tater). These adapters recognize specific phosphotyrosine residues in the autophosphorylated receptor. Src itsetf is not a receptor, but it is a tyrosine kinase. It has an SH2 and SH3 domain to tink it to the receptor and, when this occurs, it becomes activated as a tyrosine kinase. 

Phosphohpases of type Cy are activated by receptor tyrosine kinases (see Chapter 8), and thus phosphohpase Cy is involved in growth factor controlled signal transduction pathways. The receptor tyrosine kinases (see Chapter 8) phosphorylate the enzyme at specific tyrosine residues and initiate activation of the enzyme. Characteristic for the structure of phospholipase Cy is the occurrence of SH2 and SH3 domains (see Chapter 8). These represent protein modules that serve to attach further partner proteins. 

In Fig. 8.14, the structures of some nonreceptor tyrosine kinases are shown schematically (review Taniguchi, 1995). Tire nonreceptor tyrosine kinases can be divided into different families based on sequence homology the Src family is the best characterized. In addition to the catalytic domain, the nonreceptor tyrosine kinases often have SH2 and SH3 domains responsible for specific association with substrate proteins. Nonreceptor tyrosine kinases that contain both SH2 and SH3 domains may be involved in signal transduction in larger protein complexes. 

These observations and mutation studies have shown that the activity of c-Src kinase is subject to strict regulation. The domain structure of c-Src kinase is shown in Fig. 8.12. Src kinase carries a myristinic acid residue as a membrane anchor and is regulated in a complex manner by SH2 and SH3 domains and via Tyr phosphorylation (see Fig. 8.12). 

The crystal structure clearly shows that Src kinase is held by intramolecular interactions in an inactive conformation, in which the binding surfaces of the SH2 and SH3 domains are sequestered. Several paths for conversion to the active, open form are under discussion. Ibis could take place by activation via dephosphorylation of the C-terminal tail. It could also take place by the apposition of a high affinity ligand for the SH2/SH3 domains. In addition, phosphorylation of Tyr416 is also needed for full activation. 

The Stat proteins have SH2 and SH3 domains, a DNA binding domain and a C-ter-minal domain required for transcription activation. The activating phosphorylation takes place for Statl on Tyr701 in the vicinity of the C-terminus. In the imphosphoryla-ted form, the Stat proteins exist as monomers, whereas in the phosphorylated form, they are dimers. 

The integrins do not have any enzyme activity in their own cytoplasmic domain, but on hgand binding, stimulation of tyrosine phosphorylation is observed on the cytoplasmic side of many cells, such as fibroblasts and platelets. The exact configuration of protein-protein interactions on the cytosolic side of the integrins is not clear and the mechanism of stimulation of protein tyrosine kinases is unknown. Some components of the focal adhesion points, such as the structural protein tensin, have SH2 and SH3 domains that may serve as specific attachment points for tyrosine kinases and other signal proteins. 

---