Continuous directed evolution of proteins with improved soluble expression

We report the development of soluble expression phage-assisted continuous evolution (SE-PACE), a system for rapidly evolving proteins with increased soluble expression. Through use of a PACE-compatible AND gate that uses a split-intein pIII, SE-PACE enables two simultaneous positive selections to evolve proteins with improved expression while maintaining their desired activities. In as little as three days, SE-PACE evolved several antibody fragments with >5-fold improvement in expression yield while retaining binding activity. We also developed an activity-independent form of SE-PACE to correct folding-defective variants of maltose-binding protein (MBP) and to evolve variants of the eukaryotic cytidine deaminase APOBEC1 with improved expression properties. These evolved APOBEC1 variants were found to improve the expression and apparent activity of Cas9-derived base editors when used in place of the wild-type cytidine deaminase. Together, these results suggest that SE-PACE can be applied to a wide variety of proteins to rapidly improve their soluble expression.