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 filament in the giant sarcomere of crayfish claw closer muscle (Fukuzawa et al., 2001 EMBO J 20: 4826-4835). I-Connectin can be extended up to 3.5 microns 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 SEK repeats. In the present study, the force measurement of the single recombinant SEK polypeptide containing biotinylated BDTC and GST tags at the N and C termini, respectively, were performed by intermolecular force microscopy (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.