A Molecular and Structural Mechanism for G Protein-mediated Microtubule Destabilization

The heterotrimeric, G protein-coupled receptor-associated G protein, G alpha(s), binds tubulin with nanomolar affinity and disrupts microtubules in cells and in vitro. Here we determine that the activated form of G alpha(s) binds tubulin with a K-D of 100 nM, stimulates tubulin GTPase, and promotes microtubule dynamic instability. Moreover, the data reveal that the alpha 3-beta 5 region of G alpha(s) is a functionally important motif in the G alpha(s)-mediated microtubule destabilization. Indeed, peptides corresponding to that region of G alpha(s) mimic G alpha(s) protein in activating tubulin GTPase and increase microtubule dynamic instability. We have identified specific mutations in peptides or proteins that interfere with this process. The data allow for a model of the G alpha(s)/tubulin interface in which G alpha(s) binds to the microtubule plus-end and activates the intrinsic tubulin GTPase. This model illuminates both the role of tubulin as an "effector" (e.g. adenylyl cyclase) for G alpha(s) and the role of G alpha(s) as a GTPase activator for tubulin. Given the ability of G alpha(s) to translocate intra-cellularly in response to agonist activation, G alpha(s) may play a role in hormone- or neurotransmitter-induced regulation of cellular morphology.