Opposing effects of Ndel1 and α1 or α2 on cytoplasmic dynein through competitive binding to Lis1

Lis1 is an essential protein whose insufficiency causes aberrant neuronal positioning during neocortical development. It is believed to regulate both cytoplasmic dynein, a microtubule minus-end-directed motor, through direct interaction, and platelet-activating factor acetylhydrolase (PAF-AH) Ib by complexing with the catalytic subunits α1 and α2. Although α1 and α2 are highly expressed in brain, their deficiencies fail to cause brain abnormality. Here, we show that overexpression of α2 or α1 results in inactivation of dynein characterized by Golgi and endosome dispersion and mitotic delay. Further overexpression of Lis1 or Ndel1, a Lis1- and dynein-binding protein that is also crucial for dynein function, restored Golgi and endosome distribution. Biochemical assays showed that α1 and especially α2, were able to compete against Ndel1 and dynein for Lis1 binding in a dose-dependent manner. Overexpression of α2 in developing rat brain repressed the radial migration of neurons and mitotic progression of neuroprogenitors. By contrast, a Lis1-binding-defective point mutant, α2^E39D, was ineffective in the above assays. These results indicate an antagonistic effect of α1, α2 and Ndel1 for Lis1 binding, probably to modulate dynein functions in vivo. They also help to explain why brain development is particularly sensitive to a decrease in Lis1 levels.