Tubulin-G protein interactions involve microtubule polymerization domains

It has been suggested that elements of the cytoskeleton contribute to the signal transduction process and that they do so in association with one or more members of the signal-transducing G protein family. Relatively high-affinity binding between dimeric tubulin and the alpha subunits of Gs and Gi1 has also been reported. Tubulin molecules, which exist in solution as alpha beta dimers, have binding domains for microtubule-associated proteins as well as for other tubulin dimers. This study represents an attempt to ascertain whether the association between G proteins and tubulin occurs at one of these sites. Removal of the binding site for MAP2 and tau from tubulin by subtilisin proteolysis did not influence the association of tubulin with G protein, as demonstrated in overlay studies with [125I]tubulin. A functional consequence of that association, the stable inhibition of synaptic membrane adenylyl cyclase, was also unaffected by subtilisin treatment of tubulin. However, ring structures formed from subtilisin-treated tubulin were incapable of effecting such inhibition. Stable G protein-tubulin complexes were formed, and these were separated from free tubulin by Octyl-Sepharose chromatography. Using this methodology, it was demonstrated that assembled microtubules bound G protein quite weakly compared with tubulin dimers. The alpha subunit of Gi1 and, to a lesser extent, that of Go were demonstrated to inhibit microtubule polymerization. In aggregate, these data suggest that dimeric tubulin binds to the alpha subunits of G protein at the sites where it binds to other tubulin dimers during microtubule polymerization. Interaction with signal-transducing G proteins, thus, might represent a role for tubulin dimers which is independent of microtubule formation.