Introduction
Starting in 1900, amylin deposits were first discovered in pancreatic islet cells in diabetic patients. Later in 1943, more characterization was done and it was determined that these amylin deposits were amyloid in nature. More research was done over the years and by the 1980s, amylin was properly identified and the 37 amino acid sequence was identified. By 1995, the first analog of amylin, pramlintide, was synthesized. In the late 1990s, it was discovered through cryogenic electron microscopy that the amylin receptor was made of the calcitonin receptor core. The calcitonin heterodimerizes with a receptor-activating modifying protein, or RAMP, to form different amylin receptors.[1]
Figure 1. Sequence alignment comparing the amylin residues across species.
[1] Amylin
Amylin is extremely conserved among species, in order to maintain proper structure and function. Some of the main are Lysine 1, Cysteine 2, Alanine 5, Threonine 6, and Cysteine 7. All of the conserved residues exhibit extensive hydrogen bonding networks between either other residues or surrounding water molecules. There is also a disulfide bond between that is conserved across almost every species. This disulfide causes the N-terminus of amylin to wrap around. [1]
T9
Threonine 9 is an essential residue for the stabilization of amylin in the receptor. Threonine side chains are polar which allows them to hydrogen bond with other nearby polar groups, which can lead to extensive networks of interactions. This is seen in amylin at T9. T9 interacts with the of Y191, M230, I380, and H381 of the calcitonin receptor and many surrounding water molecules, but it also interacts with the of S159, N194, S195, H226, N233, and Q383. All of these interactions create a very strong interaction between amylin and the receptor. The water network also helps stabilize the active receptor conformation. [2]
Figure 2. RAMP 1, 2, and 3 superimposed in the calcitonin receptor. RAMP 1 is in pink, RAMP 2 is in tan, RAMP 3 is in blue, calcitonin receptor is in grey, and amylin is in yellow. pdb: 7TYF (RAMP 1), 7TYX (RAMP 2), 7TZF (RAMP 3).
