Coiled Coil in the Stalk Region of ncd Motor Protein Is Nonlocally Sustained

The dimeric structure of kinesin superfamily proteins plays an important role in their motile functions and characteristics. In this study, the coiled-coil-forming property of the stalk region (192-346) of Drosophila ncd, a C-terminal kinesin motor protein, was investigated by synthesizing various peptide fragments. The alpha helicity of a set of 46-residue peptides spanning the stalk region appeared too low to form a coiled-coil dimer, probably because of insufficient continuity of the hydrophobic residues at (a and d) core positions in amphipathic heptad repeats. On the other hand, several peptides with leucine residues introduced at core positions or with extensional sequences with high alpha helicity had an advantage in coiled-coil formation. When we analyzed the thermal and urea-induced unfolding of these dimeric peptides, we identified four domains having a relatively high potential to form coiled coils. Among them, three domains on the C-terminal side of the stalk region, i.e., (252-272), (276-330), and (336-346), were in the same heptad frame, although these potential coiled-coil domains were not self-sustaining individually. This is in sharp contrast to the fragment of human kinesin, (332-369), which has an extremely high tendency toward coiled-coil formation. One of the possible triggers for coiled-coil formation of the ncd stalk region may be the interaction between the motor domain and the C-terminal part of the stalk as previously revealed by X-ray crystallography. The residues, S331 and R335, seem to act as a breaking point for alpha-helix continuity. This would make the region (336-346), as the head-stalk joint, more flexible such as seen with a plus-end-directed kinesin, if this region had no interaction with the motor domain. These characteristic differences between ncd and kinesin suggest that the nonlocally sustained coiled coil of ncd is one of the factors important for minus-end-directed motility.