Transglutaminase substrates for labeling

A recombinant KalbTGase substrate comprising a fusion polypeptide of Formula I,

• wherein
• a hyphen “-” denotes a peptide bond;
• n is an integer number from 1 to 6;
• a pair of square brackets “[” and “]” delimits N-terminal and C-terminal borders of an amino acid sequence which is present n times;
• An is a FKBP chaperone amino acid sequence, and in case of n>1, each An is an independently selected FKBP chaperone amino acid sequence;
• Ln is a coupling amino acid sequence, and in case of n>1, each Ln is an independently selected coupling amino acid sequence,
• B is a target polypeptide amino acid sequence;
• and wherein
• each Ln is of Formula IIa or Formula IIb
• wherein
• m is an integer number from 1 to 10;
• a pair of square brackets “[” and “]” delimits N-terminal and C-terminal borders of an amino acid sequence which is present m times;
• T is a flexible or rigid linker amino acid sequence comprising 5 to 500 amino acids;
• Rm is a flexible or rigid linker amino acid sequences comprising 5 to 500 amino acids, and in case of m>1, each Rm is an independently selected flexible or rigid linker amino acid sequences comprising 5 to 500 amino acids;
• Qtagm is an amino acid sequence motif containing an acyl donor glutamine residue for KalbTGase transglutaminase activity, and in case of m>1, each Qtagm is an independently selected amino acid sequence motif containing an acyl donor glutamine residue for KalbTGase transglutaminase activity;
• with the proviso that SEQ ID NO:
• 18 and SEQ ID NO:
• 19 are excluded from Formula I.