Kwon, S.; Andreas, M. P.; Jones, J. A.; Giessen, T. W.

The cytosolic delivery of therapeutic proteins remains one of the most persistent challenges in modern drug delivery. Here, we report the discovery and characterization of an encapsulin-based protein nanocage, QtEnc, with unexpected permeability properties and the ability to internalize cargo proteins in vitro, fundamentally departing from existing protein nanocage cargo loading paradigms. This permeability enables simple, rapid, and single-step post-assembly cargo loading, accommodating cargos as large as 482 kDa, and allowing multiplexed cargo co-encapsulation with tunable ratios. Leveraging this property, we develop a modular QtEnc-based NanoCarrier (QtEncNC) with a pH-responsive cargo detachment module and an endosomal escape module, enabling low pH-triggered cargo release from assembled shells and subsequent endosomal escape for cytosolic delivery. Using a cytotoxic protein, BLF1, as a proof-of-concept QtEncNC payload, we demonstrate efficient cytosolic protein delivery in HeLa cells. These findings establish QtEncNC as a versatile and modular platform for cytosolic protein delivery with broad biomedical potential.