caused by several features; the prolines have a shape that causes to chain to turn, and especially prevents alpha helices. Also, the proteins contain many positively charged residues that repel each other. This results in loose proteins that are suitable as binding proteins. These binding interactions can be hydrophobic interactions as proline-rich proteins tend to have exposed hydrophobic regions. The prolines themselves work as additional binding sites for hydrogen bonds by being strong hydrogen acceptors. References Salivary proline-rich proteins