Incorporation and channel formation of human calcitonin in phosphatydilcholine planar membranes as a function of pH

Human calcitonin (hCt) is a 32 amino acid peptide contributing to calcium homeostasis. CD studies indicate that hCt has a lower propensity to form secondary structures than salmon calcitonin (sCt) (Epand et al., 1985, Int. J. Pept. Protein Res., 25, 105). This can cause fibrillation, which is critically influenced by pH, as Lys 18 protonation and Asp 15 deprotonation occur at different pH values (Kamihira et al., 2000, Protein Sci., 9, 867). However, hCt forms channels in planar lipid membranes of palmitoyloleoylphosphatidylcholine: dioleoylphosphatidylglycerol (85:15) at low concentrations or at high applied voltages (Stipani et al., 2001, Biophysical J., 81, 3332), although the biological relevance of this action has not yet been demonstrated. On the other hand, hCt fails to interact with POPC or egg phosphatidylcholine (Epand et al., 1983, Biochemistry, 22, 504), nor does it form channels in palmitoylphosphatidylcholine planar membranes (Stipani et al., 2001, Biophysical J., 81, 3332). We have studied the interaction of hCt with planar membranes made up of palmitoyloleoylphosphatidylcholine at pH 7, pH 3.8 and pH 3.12, in a KCl (1M) medium. Our results show that hCt easily incorporates into membranes and forms channels at low pH with a central conductance similar to that obtained in palmitoyloleoylphosphatidylcholine:dioleoylphosphatidylglycerol membranes. Furthermore, the time lag between the addition of hCt and the first channel appearance is dramatically reduced, while the number of channels/min increases substantially. At pH 7, even when left for as long as 24 hours in the bathing medium, hCt failed to incorporate and form channels in the membrane. These results confirm that alpha-helical conformation is the main driving force for peptide incorporation into membranes, thus preventing molecules from forming fibrils.