Pro-rich domains

Fig. 9.8. Domain structure of mSos. The mSos-1 protein of mammals possesses a pleckstrin homology domain (PH), a Pro-rich domain for interaction with Grb2 and a catalytic domain with three sequence motifs (1,2,3) characteristic for Ras GEFs.

SH3 domains occur in signal proteins that are involved in Tyr kinase signaling pathways (see Cohen et al., 1995 Pawson, 1995). They are also foimd in proteins of the cytoskeleton and in a subunit of the neutrophilic cytochrome oxidase. Ligand binding at SH3 domains takes place via Pro-rich sequences of ca. 10 amino acids, and Pro-rich peptides are very good binding substrates. 

Hie structural determination of SH3 domains with bound Pro-rich peptides has shown that the Pro-rich section of the ligand is bound as a left-handed polyproline Type 11 hehx with three amino acid residues per turn. Hie polyproline Type 11 hehx was described for polyproline some time ago. 

The sequence X-P-p-X-P is a consensus sequence for SH3 ligands, in which the two proline residues P are invariant, X is usually an aliphatic residue and p is often a Pro residue. The two invariant proline residues are each bound in a hydrophobic pocket of the SH3 domain. Peptide hgands can be bound in the C- N and also in the N- C direction. Like the SH2 domains, there are many different SH3 domains. The different SH3 domains demonstrate differing binding preferences for Pro-rich sequences, the specificity being determined by the neighboring residues of the invariant proline. 

Fig. 8.13. SH3 domains and specificity of tyrosine kinases. Binding of the SH3 domain of the substrate protein SI to the Pro-rich sequence of the tyrosine kinase increases affinity of the tyrosine kinase for its substrate and favors its phosphorylation. Proteins (S2) that possess Tyr phosphorylation sites but do not have SH3 domains are not converted by the tyrosine kinase, or only to a much lower extent.

The structure of the mSos protein (m=mammalian) is shown in Fig. 9.8. Sequence comparison with known Ras-specific GEFs has identified a common domain of ca.200 amino acids, to which nucleotide exchange activity has been assigned. Within this domain, three highly conserved sequence elements can be differentiated, separated by more variable sections. Other structural elements include a PH domain and a Pro-rich binding domain. The Pro-rich sequence fimctions as an attachment site for the SH3 group of Grb2 protein. 

H189E <1> (<1> comparable to wild type enzyme [26] <1> thermodynamics [29] <1> mutant retains ability for phosphorylation [34]) [26, 29, 34] Additional information <1, 3, 6, 12> (<1,3,6> overview on mutants [19] <1> N-terminal domain, thermodynamic properties [25,29] <3,6,12> enzyme deletion mutants, virulence of [31] <6> enzyme deletion mutant, alternative pathways [36] <1> fusion protein of enzyme plus the remaining three subunits of glucose phosphotransferase system and Ala-Pro-rich linker sequences [32]) [19, 25, 29, 31, 32]... 

Tokumitsu, H., Takahashi, N., Eto, K., Yano, S., Soderling, T. R. and Muramatsu, M., 1999, Substrate recognition by Ca2+/Calmodulin-dependent protein kinase kinase. Role of the arg-pro-rich insert domain, J Biol Chem, 274, pp 15803-10. 

Kunst et al., 2000). At the N-terminal end of skeletal C-protein is a sequence rich in proline and alanine, and therefore known as the Pro-Ala domain. Cardiac C-protein has an additional Ig domain (CO) at its N-terminus and has some sequence insertions both in the S2-binding region, which can also be phosphorylated, and in C5. It has been found that interactions can occur between domains 5 and 8 and between domains 7 and 10 (Fig. 21C) of different C-protein molecules (Moolman-Smook et al, 2002), and these interactions were used to suggest that three C-protein molecules form a collar around the myosin filament. 

Like the SH2 domains, there are many different SH3 domains. The different SH3 domains demonstrate differing binding preferences for Pro-rich sequences, the specificity being determined by the neighboring residues of the invariant proline. 

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