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The regulatory domain of myosin consists of a heavy-chain fragment (HC), the ELC and the RLC. The ELC wraps around the N-terminal end of the HC, while the RLC binds to the C-terminal end. The N-terminal domain EF-hands (EF1, EF2) of ELC are in the closed conformation (like Ca2+-free CaM and TnC); it is interesting to note that only ELC EF1 binds Ca2+, yet it still forms a closed conformation. ELC EF2 is rather unusual in its position, and is the only EF-hand of the light chains not making contact with the HC. The N-terminal domain EF-hands of RLC are in the open conformation (Ca2+-CaM, Ca2+-TnC), and RLC EF1 binds a Mg2+ ion. The C-terminal domain EF-hands (EF3, EF4) of both light chains - none of which bind metal ions - assume a semi-open conformation in which the helices are not as far apart as in the open conformation, not as close together as in the closed conformation. However, this conformation is not characterised by uniform values of phi, theta and omega as in the open and closed conformations, nor do the values indicate that the semi-open conformation is an intermediate between the two. |
| EF-hand (helix residues) | N-terminal coordinate
of second helix |
theta | phi | omega |
| ELC EF1 (11-18, 30-40) | (11.192, -0.587, -2.606) | 42.224 | 118.519 | 112 |
| ELC EF2 (46-53, 65-75) | (2.353, 11.068, -7.121) | 43.166 | -160.151 | 23 |
| ELC EF3 (86-93, 103-113) | (10.557, -3.027, -4.187) | 62.039 | 75.110 | 167 |
| ELC EF4 (122-129, 141-151) | (12.036, -0.420, -2.217) | 90.230 | 123.055 | 144 |
| RLC EF1 (20-27, 37-47) | (6.755, -0.829, -8.174) | 85.201 | 106.902 | 81 |
| RLC EF2 (56-63, 69-79) | (9.145, 2.622, -6.354) | 84.804 | 127.200 | 80 |
| RLC EF3 (89-96, 106-116) | (10.015, -5.325, -4.848) | 73.995 | 62.202 | 196 |
| RLC EF4 (125-132, 141-151) | (7.449, 6.707, -5.399) | 83.304 | 135.244 | 158 |
NB: values obtained using apo-CaM EF1 (as in refcam.pdb) as the reference EF-hand.