Hydrophobic domains of human tropoelastin interact in a context-dependent manner

Tropoelastin is the soluble precursor of elastin, the major component of th e extracellular elastic fiber. Tropoelastin undergoes self-association via an inverse temperature transition termed coacervation, which is a crucial s tep in elastogenesis. Coacervation of tropoelastin takes place through mult iple intermolecular interactions of its hydrophobic domains. Previous work has implicated those hydrophobic domains located near the center of the pol ypeptide as playing a dominant role in coacervation. Short constructs of do mains 18, 20, 24, and a mutated form of domain 26 were largely disordered a t 20 degreesC but displayed increased order on heating that was consistent with the formation of beta -structures. However, their conformational trans itions were not sensitive to physiological temperature in contrast to the o bserved behavior of the native domain 26. A polypeptide consisting of domai ns 17-27 of tropoelastin coacervated at temperatures above 60 degreesC, whe reas individually expressed hydrophobic regions were not capable of coacerv ation. We conclude that coacervation depends on the hydrophobicity of the m olecule and, by inference, the number of hydrophobic domains. Tropoelastin mutants were constructed to contain a Pro --> Ala mutation in domain 26, se parate deletions of domains 18 and 26, and a displacement of domain 26. The se constructs displayed unequal capacities for coacervation, even when they contained the same number of hydrophobic regions and comparable levels of secondary structure. Thus, the capability for coacervation is determined by contributions from individual hydrophobic domains for which function shoul d be considered in the context of their positions in the intact tropoelasti n molecule.