A different conformation for linker L12 in IF molecules in the molecular and filamentous forms: an hypothesis

The rod domain of IF molecules has been characterized as four alpha-helical coiled-coil segments (1A, 1B, 2A and 2B), three linkers (L1, L12 and L2) and a stutter at the centre of segment 2B. Two of these breaks in coiled-coil continuity (L2 and stutter) have been modelled on the basis of structural data obtained from related proteins. Subsequently, X-ray crystallographic studies on fragments of IF molecules have shown that both models were correct. The third of the breaks - L1 - was predicted to have a flexible structure, consistent with observations that the head domain can fold back over segments 1A and 1B and also unwind into separate strands. Here the structure of the fourth discontinuity (L12) has been modelled. For most IF chain types two conformations are proposed for an eight-residue motif that displays a quasi two-residue repeat based on the presence of apolar residues. In IF it is proposed that the motif will adopt an alpha-helical conformation but that in the molecule the conformation will be beta-like. Thus, assembly will result in or result from a conformational change in L12 thereby attributing L12 a more dynamic and important role in assembly than expected.