Electrostatic Self-Assembly of Protein Nanoparticles from Intrinsically Disordered Polypeptide and Globular Protein Pairs.

Intracellular delivery of folded, functional proteins remains a critical barrier to realizing the full potential of protein therapeutics. We present a fully aqueous, genetically encoded platform for assembling micellar protein nanoparticles via electrostatic coacervation between anionic globular proteins and multi-domain intrinsically disordered proteins (IDPs) composed of a neutral elastin-like polypeptide (ELP) domain and a cationic disordered histone-derived domain (H5). This biosynthetic system allows modular control over charge density, neutral domain length, and stoichiometry, enabling the formation of homogeneous micellar nanoparticles with high protein payload retention under mild, biologically relevant conditions without organic solvents or covalent modification. Unlike amphiphilic micelle systems, globular proteins in this approach act as both cargo and assembly drivers, providing a direct handle to tune encapsulation. Nanoparticle assembly was characterized with a suite of complementary techniques including DLS, FCS, TEM, and SAXS, enabling detailed analysis of nanoparticle structure, size, and composition. The system exhibits remarkable tolerance to different formulations, consistently producing well-defined particles with low dispersity. This expanded design space arises from the use of structured, charge-regulating proteins and confers exceptional versatility in formulation. Different charge fractions modulate not only nanoparticle size but also physicochemical properties such as core density and cargo loading, which may be critical for tuning nanoparticle stability and performance. Together, these features establish a robust and programmable platform for protein-based nanoparticle engineering with potential application in intracellular protein delivery.