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TNC induces a conformational change, which is transmitted to the TNI bound to the C-
lobe of TNC. TN1 then releases actin and tropomyosin is displaced, allowing interaction
between actin and myosin. Calcium binding in TNC is therefore the trigger for striated
muscle contraction in response to Ca 2+ influx from the sarcoplasmic reticulum.
Houdusse et al. explain that the Troponin C structure starts with an N-terminal helix
named helix N which is specific to TNC. This is followed by the first of four helix-turn-
helix Ca 2+ binding domains, called EF hands. It is comprised of helices A and B which
are separated by a loop. The latter contains a hinge spanning residues 32 to 38. A linker
connects helix B to C, and then a loop containing a hinge at residue 70 connects C and D,
forming EF hand II. Helices B and C move together as a result of flexure about these two
hinges, leaving helices N,A, and D virtually unmoved. Helix D is part of a larger helix
which includes the helical linker connecting the N- and C-lobes and helix E of the C-
lobe. In the C lobe, a hinge appears at residues 100 and 101 between helices E and F,
forming EF hand III. The structure is completed by EF hand IV, comprised of helices G
and H, which show less significant hinge motion, possibly because they are bound to
calcium in both 2TN4 and 4TNC, the two structures compared in this study.
The HAG annotation (hinges at residues 38,39,65,66,104 and105) differs slightly from
the above hinge locations. The first two hinge points differ mostly because of differing
hinge selection criteria. The hinges at 104 and 105 actually align exactly with the
mentioned residues 100 and 101, the difference reflecting only residue numbering
differences between the Houdousse et al.’s holo structure (PDB ID 2TN4, morph ID
201