Proline Periodicity Modulates the Self-Assembly Properties of Elastin-Like Polypeptides

Abstract

Elastin is a self-assembling protein of the extracellular matrix that provides tissues with elastic extensibility and recoil. The monomeric precursor, tropoelastin, is highly hydrophobic yet remains substantially disordered and flexible in solution, due in large part to a high combined threshold of proline and glycine residues within hydrophobic sequences. In fact, proline-poor elastin-like sequences are known to form amyloid-like fibrils, rich in β-structure, from solution. On this basis, it is clear that hydrophobic elastin sequences are in general optimized to avoid an amyloid fate. However, a small number of hydrophobic domains near the C-terminus of tropoelastin are substantially depleted of prolines. Here we investigated the specific contribution of proline number and spacing to the structure and self-assembly propensities of elastin-like polypeptides. Increasing the spacing between proline residues significantly decreased the ability of polypeptides to reversibly self-associate. Characterization of the assembly process revealed the presence of smaller colloidal droplets with enhanced propensity to cluster into dense networks, enriched in β-structure. These data strongly support a model where proline-poor regions of the elastin monomer provide a unique contribution to assembly, and suggest a role for localized β-sheet in mediating self-assembly interactions.