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Directed Evolution Method in Saccharomyces cerevisiae: Mutant Library Creation and Screening
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The VEGAS Platform Is Unsuitable for Mammalian Directed Evolution.

Christopher E Denes1, Alexander J Cole2, Minh Thuan Nguyen Tran3

  • 1The Dr. John and Anne Chong Lab for Functional Genomics, Charles Perkins Centre and School of Life & Environmental Sciences, The University of Sydney, Sydney, NSW 2006, Australia.

ACS Synthetic Biology
|October 11, 2022
PubMed
Summary

The viral evolution of genetically actuating sequences (VEGAS) system for mammalian directed evolution has critical flaws. It fails to support viral replication and generates "cheater" particles, preventing effective protein evolution.

Keywords:
Sindbis virusVEGASdirected evolutionreplicabilityreproducibility

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Area of Science:

  • Biotechnology
  • Molecular Biology
  • Protein Engineering

Background:

  • Directed evolution is a powerful technique for protein engineering, traditionally performed in prokaryotic systems.
  • A recently described mammalian system, viral evolution of genetically actuating sequences (VEGAS), aimed to extend directed evolution to mammalian cells.

Purpose of the Study:

  • To evaluate the efficacy and limitations of the VEGAS system for mammalian directed evolution.
  • To identify the underlying causes of system failure and assess its potential for future development.

Main Methods:

  • Utilized a deconstructed Sindbis virus (SINV) genome as the basis for the VEGAS system.
  • Assessed viral replication, transgene stability, and particle composition through sequencing and functional assays.

Main Results:

  • The VEGAS system failed to support Sindbis structural gene (SSG)-dependent viral replication.
  • Viral particles generated by VEGAS rapidly lost target transgenes, with "cheater" particles containing SINV structural RNA dominating.
  • Sequencing revealed contamination from packaging RNA, not VEGAS-derived RNA, indicating a fundamental flaw in particle generation and selection.

Conclusions:

  • The VEGAS system, as currently described, possesses significant limitations that prevent its use for mammalian directed evolution.
  • Defective SSG complementation and the prevalence of "cheater" particles preclude effective selection of authentic evolution products.
  • Substantial redesign is required to overcome these issues and enable functional mammalian directed evolution using this approach.