Single-molecule measurement of elasticity of Serine-, Glutamate- and Lysine-Rich repeats of invertebrate connectin reveals that its elasticity is caused entropically by random coil structure
Invertebrate connectin (I-connectin) is a 1960 kDa elastic protein linking the Z line to the tip of the myosin filamentin the giant sarcomere of crayfish claw closer muscle (Fukuzawa et al., 2001 EMBO J 20: 4826–4835). I-Connectincan be extended up to 3.5 μm upon stretch of giant sarcomeres. There are several extensible regions in I-connectin:two long PEVK regions, one unique sequence region and Ser-, Glu- and Lys-rich 68 residue-repeats called SEKrepeats. In the present study, the force measurement of the single recombinant SEK polypeptide containingbiotinylated BDTC and GST tags at the N and C termini, respectively, were performed by intermolecular forcemicroscopy (IFM), a refined AFM system. The force vs. extension curves were well fit to the wormlike chain (WLC)model and the obtained persistence length of 0.37±0.01 nm (n=11) indicates that the SEK region is a random coil along its full length. This is the first observation of an entropic elasticity of a fully random coil region that contributes to the physiological function of I-connectin.
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