Homology of calcitonin with the amyloid-related proteins

We present evidence for a structural homology between the amino acid sequence of calcitonin (CT) — the fibrillar protein of the amyloid deposits of medullary thyroid cancer — and that of other 12 amyloid-related proteins (ARP). Seven of the 32 residues of CT are conserved in at least five ARP, and five of these seven amino acids belong to the stretch Gly2-Gln14. Gln14 is conserved in all 12 ARP and Cys7 in all eight ARP containing cysteine. The concentration of the homology in the N-terminal half of CT goes along with the knowledge that is the C-terminal region the one more important for the hormonal action of CT. Since an imbalance between synthesis and catabolism of a given ARP is believed to be the general pathogenetic mechanism of amyloidosis, the intratumoral deposition of CT in the form of amyloid fibrils would be due to the overproduction of a protein structurally similar to the ARP.