Bifunctional fusion proteins containing the sequence of the bradykinin homologue maximakinin: activities at the rat bradykinin B 2 receptor.

To support bradykinin (BK) B2 receptor (B2 R) detection and therapeutic stimulation, we developed and characterized fusion proteins consisting of the BK homolog maximakinin (MK), or variants, positioned at the C-terminus of functional proteins (enhanced green fluorescent protein (EGFP), the peroxidase APEX2, or human serum albumin (HSA)). EGFP-MK loses its reactivity with anti-BK antibodies and molecular mass as it progresses in the endosomal tract of cells expressing rat B2 Rs (immunoblots, epifluorescence microscopy). APEX2-(NG)15 -MK is a bona fide agonist of the rat, but not of the human B2 R (calcium and c-Fos signaling) and is compatible with the cytochemistry reagent TrueBlue (microscopy), a luminol-based reagent, or 3,3',5,5'-tetramethylbenzidine (luminescence or colourimetric B2 R detection, cell well plate format). APEX2-(NG)15 -MK is a non-isotopic ligand suitable for drug discovery via binding competition. Affinity-purified secreted forms of HSA fused with peptides possessing the C-terminal MK or BK sequence failed to stimulate the rat B2 R in the concentration range of 50-600 nmol/L. However, the non-secreted construction myc-HSA-MK is a B2 R agonist, indicating that protein denaturation made the C-terminal sequence available for receptor binding. Fusion protein ligands of the B2 R are stable but subjected to slow intracellular inactivation, strong species specificity, and possible steric hindrance between the receptor and large proteins.