Signaling function of alpha-catenin in microtubule regulation.

Centrosomes control microtubule dynamics in many cell types, and their removal from the cytoplasm leads to a shift from dynamic instability to treadmilling behavior and to a dramatic decrease of microtubule mass (Rodionov et al., 1999; PNAS 96:115). In cadherin-expressing cells, these effects can be reversed:non-centrosomal cytoplasts that form cadherin-mediated adherens junctions display dense arrays of microtubules (Chausovsky et al., 2000; Nature Cell Biol 2:797). In adherens junctions, cadherin's cytoplasmic domain binds p120 catenin and beta-catenin, which in turn binds alpha-catenin. To elucidate the roles of the cadherin-associated proteins in regulating microtubule dynamics, we prepared GFP-tagged, plasma membrane targeted or untargeted p120 catenin, alpha-catenin and beta-catenin and tested their ability to rescue the loss of microtubule mass caused by centrosomal removal in the poorly adhesive cell line CHO-K1. Only membrane targeting of alpha-catenin led to a significant increase in microtubule length and density in centrosome-free cytoplasts. Expression of non-membrane-targeted alpha-catenin produced only a slight effect, while both membrane-targeted and non-targeted p120 and beta-catenin were ineffective in this assay. Together, these findings suggest that alpha-catenin is able to regulate microtubule dynamics in a centrosome-independent manner.