Made-To-Order Branched Proteins

Cells usually build proteins in strings that then fold up into three-dimensional shapes. Some protein engineers, however, would like to construct proteins that branch like synthetic polymers do. These new protein shapes could lead to novel biomaterials with applications such as drug delivery or encapsulating cells in tissue engineering.

Now chemists demonstrate that simple, genetically encoded tags can make proteins branch in a variety of shapes (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja4076452).

The team, led by Frances H. Arnold and David A. Tirrell at California Institute of Technology, read about the tags in a 2012 paper (Proc. Natl. Acad. Sci. USA 2012, DOI: 10.1073/pnas.1115485109). A British group produced the tags from a bacterial protein that forms an unusual bond, called an isopeptide bond, between the side chains of a lysine and an aspartate. They split the gene for the protein in two so that, when translated, one peptide contained the lysine and another had the aspartate. When mixed, the two peptides, called SpyTag and SpyCatcher, link up, forming the isopeptide bond in minutes. The two tags can be inserted into the genes of separate proteins and act like a dab of protein cement to connect the biomolecules.

The Caltech group quickly saw the potential to build unique protein shapes by placing the tags in different positions in a protein chain. For example, the team made tadpole-like shapes by inserting SpyCatcher at one end of an engineered peptide and SpyTag in the middle of the same chain. So far, they’ve made circles, stars, and H-shapes. The branching reactions are simple and efficient, the researchers say, occurring spontaneously in water or living cells without external catalysts.

The tags, Tirrell says, provide “a way to get beyond linear proteins and take advantage of various branching phenomena that have been used to such good effect in polymer chemistry.”