The PROTEin Evolution Using Selection (PROTEUS) platform represents a major breakthrough in biological engineering. It enables robust directed evolution of biomolecules directly within mammalian cells. Traditional methods often face challenges like instability, safety concerns, and limited functionality. PROTEUS solves these issues using capsid-deficient virus-like vesicles (VLVs), which provide a stable and reliable system. This innovation supports the evolution of proteins with enhanced, context-specific properties. By overcoming key obstacles in mammalian directed evolution, the platform opens new avenues for medical and biotechnological advances.
What Really PROTEUS Mean?
PROTEUS is a chimeric viral platform designed for prolonged directed evolution campaigns in mammalian cells. Directed evolution mimics natural selection through iterative cycles of diversification, selection, and amplification* to engineer biomolecules with novel or improved functions. Mammalian cells offer a natural environment that supports intricate post-translational modifications, protein-protein interactions, and signaling networks. This makes them ideal for developing proteins designed for use in mammalian applications, unlike prokaryotic or yeast-based systems. PROTEUS leverages these advantages to generate biomolecules with superior, context-specific performance.
Overcoming Limitations of Prior Systems
Historically, mammalian directed evolution faced significant hurdles:
- Host genome mutations: Early cell-based systems were prone to unintended genetic changes, compromising stability.
- Viral system limitations: Existing virus-based platforms suffered from safety risks, low mutation rates, target specificity issues, or production of non-functional “cheater particles” that undermined selection fidelity.
- Alphavirus-based challenges: Previous alphavirus systems struggled with system integrity due to recombination or loss of selective pressure.
PROTEUS resolves these issues by using capsid-deficient VLVs derived from alphaviruses. These VLVs depend on the host cellโs production of the Indiana vesiculovirus G (VSVG) coat protein for infectivity and propagation, ensuring host-dependent propagation. This design prevents the formation of cheater particles and maintains a robust link between the target biomoleculeโs activity and the VLVโs fitness, enabling continuous and precise selective pressure.
Mechanism of Diversification
PROTEUS takes advantage of the error-prone RNA-dependent RNA polymerases found in alphaviruses. These enzymes naturally create mutations often. The platform has a baseline mutation rate of 2.6 mutations per 100,000 transduced cells. It shows a strong A-to-G (and U-to-C) transition bias. This bias is mainly caused by ADAR-dependent mutations. This inherent error rate diversifies the target transgene, creating a broad pool of variants for selection.
To boost diversification, PROTEUS can include RNA nucleoside analogs such as molnupiravir. This compound increases the mutation rate by eight times, resulting in 22 mutations per 100,000 cells, and shifts the mutational bias to explore a broader array of protein variants. This tunable diversification mechanism allows PROTEUS to balance mutation rate and specificity, optimizing the evolution process for diverse applications.
Ensuring System Integrity
A hallmark of PROTEUS is its ability to maintain system integrity.The platform connects the expression level of VSVG to the activity of the viral transgene. This ensures that only working biomolecules help with VLV propagation. This tight coupling enforces continuous selective pressure, preventing the propagation of non-functional variants and maintaining the platformโs efficiency over multiple evolutionary cycles.
Demonstrated Successes
PROTEUS has proven its versatility through several high-impact applications:
Tetracycline-controlled transactivators (tTA): PROTEUS evolved tTA to become doxycycline-resistant, recovering well-characterized mutations such as R158G and the Q32R/R158G double mutant. These complex mutations, challenging to achieve through other methods, significantly enhanced resistance, demonstrating PROTEUSโs ability to generate sophisticated variants.
Reverse tetracycline-controlled transactivator (rtTA-3G): The platform optimized rtTA-3G, producing a superior variant (rtTA-4G with D5N/M59I mutations) with increased doxycycline sensitivity. This adaptation was mammalian-specific, highlighting the advantage of evolving proteins in their native cellular context, as bacterial systems could not replicate this enhancement.
ย Intracellular Nanobody (Nb139): PROTEUS transformed a p53-interacting nanobody into a biosensor, with evolved variants (S26P and Y60C) improving intracellular function, nuclear localization, and detection of p53 activity in response to DNA damage. This application underscores PROTEUSโs potential for developing advanced diagnostic tools.
Limitations and Future Directions
While PROTEUS is a groundbreaking platform, it has some limitations:
Mutational bias: The platformโs diversification is primarily ADAR-mediated, which introduces a specific A-to-G bias. While molnupiravir can modulate this bias, it reduces the overall mutation rate, requiring careful optimization.
Cell-type specificity: PROTEUS currently performs best in BHK-21 cells due to their lack of a functional Type I interferon response, which suppresses antiviral defenses. Adapting PROTEUS to other mammalian cell types may require targeted inhibition of this pathway.
VSVG expression balance: Maintaining precise VSVG expression levels for complex targets or intricate circuit designs can be challenging, potentially limiting certain applications.
Future advancements could focus on expanding PROTEUSโs compatibility with diverse cell types and refining mutation strategies to minimize bias while maximizing diversity. These improvements could further broaden the platformโs applicability in therapeutic and biotechnological contexts.
Paradigm Shift in Biomolecular Engineering
PROTEUS marks a significant leap in directed evolution, providing a stable and efficient platform specifically designed for engineering biomolecules in mammals with exceptional precision. It overcomes the limitations of earlier systems by offering customized adaptations for specific applications. This innovation paves the way for new possibilities in protein engineering, including the development of advanced therapeutics and the creation of innovative biosensors. As a cornerstone of biological innovation, PROTEUS is poised to drive the next generation of biotechnological breakthroughs.
