Sequence length controls coil-to-globule transition in elastin-like polypeptides
Phase separation of disordered proteins resulting in the formation of biocondensates has received significant attention due to its fundamental role in cellular organization and functioning and is sought after in many applications. For instance, the liquid-liquid phase separation of tropoelastin initiates the hierarchical assembly process of elastic fibers, which are key components of the extracellular matrix providing resilience and elasticity to biological tissues. Inspired by the hydrophobic domains of tropoelastin, elastin-like polypeptides (ELPs) were derived which exhibit a similar phase behavior. Even though, it appeared almost certain that elastin condensates retain liquid-like properties, a recent experimental study questioned this viewpoint by demonstrating that the aggregate state of elastin-derived materials can depend on the length of hydrophobic domains. Here, we employ state-of-the-art atomistic modeling to resolve the conformational ensembles of a single ELP as a function of its sequence length in the temperature range relevant to possible applications. For the first time, we report the free energy profiles of ELPs in the vicinity of conformational transitions which show more compact polypeptide conformations at higher temperatures in accord with their thermoresponsive nature. We access the conformations visited by ELPs through descriptors from polymer physics. We find that short ELPs always remain in coil-like conformations, while the longer ones prefer globule states. The former engages in intrapeptide hydrogen bonds temporarily retaining their liquid-like properties while the latter forms long-lived (hundreds of nanoseconds) intra-peptide hydrogen bonds attributed to ordered secondary structure motifs such as {beta}-bridges and turns. Our work demonstrates the importance of the sequence length as a modulator of conformational properties at a single chain and possibly explains the change in aggregate state in elastin condensates.