Sticky-end assembly of a designed peptide fiber provides insight into protein fibrillogenesis

Coiled-coil motifs provide simple systems for studying molecular self-assembly. We designed two 28-residue peptides to assemble into an extended coiled-coil fiber. Complementary interactions in the core and flanking ion-pairs were used to direct staggered heterodimers. These had "sticky-ends" to promote the formation of long fibers. For comparison, we also synthesized a permuted version of one peptide to associate with the other peptide and form canonical heterodimers with "blunt-ends" that could not associate longitudinally. The assembly of both pairs was monitored in solution using circular dichroism spectroscopy. In each case, mixing the peptides led to increased and concentration-dependent circular dichroism signals at 222 nm, consistent with the desired alpha-helical structures. For the designed fiber-producing peptide mixture, we also observed a linear dichroism effect during flow orientation, indicative of the presence of long fibrous structures. X-ray fiber diffraction of partially aligned samples gave patterns indicative of coiled-coil structure. Furthermore, we used electron microscopy to visualize fiber formation directly. Interestingly, the fibers observed were at least several hundred micrometers long and 20 times thicker than expected for the dimeric coiled-coil design. This additional thickness implied lateral association of the designed structures. We propose that complementary features present in repeating structures of the type we describe promote lateral assembly, and that a similar mechanism may underlie fibrillogenesis in certain natural systems.