DogTag and DogCatcher form an interacting peptide-protein pair; the DogTag peptide rapidly and spontaneously forms a covalent isopeptide amide bond with its specific binding partner, the DogCatcher protein, upon mixing. DogTag and DogCatcher can thus be used to bring together two tagged proteins of interest in vivo. The DogTag/DogCatcher pair show efficient covalent isopeptide amide bond formation both when the DogTag peptide is located at the terminus of a protein of interest, and also when the DogTag peptide is inserted internally, in a protein loop. DogTag and DogCatcher are artificial protein sequences engineered from domain 4 (residues 734-861) of the RrgA adhesin gene of Streptococcus pneumoniae; DogTag is a small peptide based on the C-terminal β hairpin of this domain, while DogCatcher is a larger protein based on the remainder of the domain (both DogTag and DogCatcher contain additional mutations/modifications relative to the RrgA sequence, to improve solubility and to optimize the rapid formation of the covalent bond between them) (PMID:34324879).