An alternative model for the structural mechanics of hagfish slime threads.

Hagfish slime threads are an interesting intermediate filament (IF) system in that, in contrast to hair, they appear to be a series system of IFs, consisting of rods and terminal domains (TDs), without matrix. The protein composition also differs from hair. Published data show that the wet stress-strain curve consists of four regions: I, increasing low modulus; II, plateau; III increasing higher modulus; IV decreasing modulus to break. Beyond 34% extension, there is plastic deformation, which contrasts with the elastic deformation of most biological fibres. The paper considers two explanations, one published by Fudge et al. [D.S. Fudge, K.H. Gardner, V.T. Forsyth, C. Riekel, J.M. Gosline, Biophys. J. 85 (2003) 2015] and one new, of deformation in the four regions. The former regards the TDs as elastic, entropic coils, which extend in region I. The latter regards the TDs as having a plastic, energy-dependent extension in regions III and IV, which is comparable to the drawing of polyester fibres. A rough theoretical model of jumps over energy barriers gives a similar prediction. Twist is suggested as a mechanism for overall thread cohesion. Experimental and theoretical ways forward to a greater understanding of the structural mechanics of hagfish threads are suggested. The behaviour of the total thread/mucus/water system is discussed and some speculations on the defensive mechanisms that have evolved are presented.