Probing the potential of CnaB-type domains for the design of tag/catcher systems.

Building proteins into larger, post-translational assemblies in a defined and stable way is still a challenging task. A promising approach relies on so-called tag/catcher systems that are fused to the proteins of interest and allow a durable linkage via covalent intermolecular bonds. Tags and catchers are generated by splitting protein domains that contain intramolecular isopeptide or ester bonds that form autocatalytically under physiological conditions. There are already numerous biotechnological and medical applications that demonstrate the usefulness of covalent linkages mediated by these systems. Additional covalent tag/catcher systems would allow creating more complex and ultra-stable protein architectures and networks. Two of the presently available tag/catcher systems were derived from closely related CnaB-domains of Streptococcus pyogenes and Streptococcus dysgalactiae proteins. However, it is unclear whether domain splitting is generally tolerated within the CnaB-family or only by a small subset of these domains. To address this point, we have selected a set of four CnaB domains of low sequence similarity and characterized the resulting tag/catcher systems by computational and experimental methods. Experimental testing for intermolecular isopeptide bond formation demonstrated two of the four systems to be functional. For these two systems length and sequence variations of the peptide tags were investigated revealing only a relatively small effect on the efficiency of the reaction. Our study suggests that splitting into tag and catcher moieties is tolerated by a significant portion of the naturally occurring CnaB-domains, thus providing a large reservoir for the design of novel tag/catcher systems.