(55c) Globular Protein Vesicles: Engineering Vesicle Size and Membrane Structure through a Tunable Molecular Packing Parameter

Vesicles containing globular, folded proteins have enormous potential for bio-functions different from traditional liposomes or polymersomes. We have designed two different types of recombinant fusion proteins, a globular domain (mCherry, enhanced green fluorescent protein (EGFP), or single chain antibody fragment (ScFv)) fused with a glutamic acid-rich leucine zipper (ZE) and an arginine-rich basic leucine zipper (ZR) fused with a thermo-responsive elastin-like polypeptide (ELP), to create globular protein vesicles through temperature-induced self-assembly. From systematic investigation of the protein-assembly process, this work suggests a simple practical strategy to engineer globular protein vesicles with desired size and membrane structure. The fusion proteins of globule-ZE and ZR-ELP form complexes via high affinity binding of the zippers, and transition through dynamic coacervate phases to stable hollow vesicles upon warming. The main parameters to decide protein vesicle size are the solution temperature and the protein concentration, which correlate with the thermal driving force behind the vesicular assembly. The thermal driving force dictates molecular packing parameter of the globule-zipper-ELP complex, which controls both of membrane curvature and structure. Unlike other vesicles, the tunable packing parameter enables formation of either single-layered or double-layered vesicles, as confirmed by localized encapsulation of hydrophobic dyes. These results provide critical information to create engineered globular protein vesicles for a number of bio-applications such as drug delivery, artificial cells, and micro-reactors.