The dimerization mechanism of LIS1 and its implication for proteins containing the LisH motif.

Miller-Dieker lissencephaly, or "smooth-brain" is a debilitating genetic developmental syndrome of the cerebral cortex, and is linked to mutations in the Lis1 gene. The LIS1 protein contains a so-called LisH motif at the N terminus, followed by a coiled-coil region and a seven WD-40 repeat forming beta-propeller structure. In vivo and in vitro, LIS1 is a dimer, and the dimerization is mediated by the N-terminal fragment and is essential for the protein's biological function. The recently determined crystal structure of the murine LIS1 N-terminal fragment encompassing residues 1-86 (N-LIS1) revealed that the LisH motif forms a tightly associated homodimer with a four-helix antiparallel bundle core, while the parallel coiled-coil situated downstream is stabilized by three canonical heptad repeats. This homodimer is uniquely asymmetric because of a distinct kink in one of the helices. Because the LisH motif is widespread among many proteins, some of which are implicated in human diseases, we investigated in detail the mechanism of N-LIS1 dimerization. We found that dimerization is dependent on both the LisH motif and the residues downstream of it, including the first few turns of the helix. We also have found that the coiled-coil does not contribute to dimerization, but instead is very labile and can adopt both supercoiled and helical conformations. These observations suggest that the presence of the LisH motif alone is not sufficient for high-affinity homodimerization and that other structural elements are likely to play an important role in this large family of proteins. The observed lability of the coiled-coil fragment in LIS1 is most likely of functional importance.