Maximakinin: An amphibian bradykinin homologue integrated into fusion proteins that bind to the bradykinin B2 receptor

Bradykinin (BK), a blood-derived nonapeptide, is a vasodilator, increases microvascular permeability and stimulates nociceptors mostly via receptors (B2Rs). Maximakinin (MK), discovered in the skin of an amphibian, has the full BK sequence extended by 9 residues at its N-terminus (DLPKINRKGPRPPGFSPFR). MK has a good affinity for the rat and rabbit B2R and is more resistant to inactivation than BK. Fusion proteins consisting of MK positioned at the C-terminus of functional proteins (enhanced green fluorescent protein (EGFP), the peroxidase APEX2) were produced as lysates of HEK 293a cells transfected with the corresponding expression vector; they are agonists of the B2R as judged from the receptor-mediated signaling in cells expressing the recombinant receptors. EGFP-MK is endocytosed along with the B2Rs and colocalized with various molecular partners (β-arrestins, Rab5, LAMP1) during its slow transition towards lysosomes (epifluorescence microscopy). It does not bind to angiotensin converting enzyme or kinin B1 receptors. The peroxidase APEX2-(Asn-Gly)₁₅-MK, containing a further spacer sequence, detects B2Rs with cytochemistry reagents, luminol or TMB. However, MK and the fusion proteins that include MK have little affinity for the human form of the B2R. Effects of changes in the spacer sequence support the feasibility of alleviating this limitation. Positioning MK or BK with spacers at the C-terminus of human serum albumin failed to produce B2R ligands. Fusion protein ligands of the B₂R are subjected to slow intracellular inactivation, species specificity and possible steric hindrance between the receptor and large proteins.