Directed Evolution Helps Build Protein Containers

By using microbial evolution as a driving force, chemists have figured out how to coax the molecular boxes that nature uses as miniature storage devices into stockpiling more stuff (Science, DOI: 10.1126/sci ence.1199081). The development, by Donald Hilvert of the Swiss Federal Institute of Technology, Zurich, and coworkers, could improve the capacity and specificity of molecular containers that are used in fields as diverse as drug delivery and catalysis. The team engineered Escherichia coli cells to produce a protein called AaLS, which spontaneously forms icosahedral shells, or capsids. The researchers also engineered AaLS to have negatively charged residues to help it store away proteins that have a positively charged tag. They then provided the E. coli with a gene that codes for a protease enzyme that is toxic to the bacteria and that has the positive tag. Electrostatic attraction helps sequester the harmful protease in the capsid, but the bacteria also have an evolutionary impetus to improve the capsid’s ability to pack away the harmful protease. When the team exposed the E. coli to millions of randomly mutated capsid genes, the bacteria incorporated improved capsids that were five to 10 times better at impounding the protease.