POLYGLUTAMYLATION OF TUBULIN AS A PROGRESSIVE REGULATOR OF IN-VITRO INTERACTIONS BETWEEN THE MICROTUBULE-ASSOCIATED PROTEIN-TAU AND TUBULIN

The multiple functions of microtubules are mediated by various structu ral and motor microtubule-associated proteins (MAPs). To harmonize the se functions in different places of a single cell, the key problem is to regulate the interactions of these proteins with microtubules. The chemical diversity of tubulin isoforms, which constitute the microtubu le wall, could represent a molecular basis for this control. Using an in vitro assay of ligand blotting, we found that the microtubule-assoc iated protein Tau interacts differentially with the diverse posttransl ationally-modified isotubulins: its binding is mainly restricted to mo derately-modified alpha- and beta-tubulin isoforms. We obtained eviden ce that the recently-discovered polyglutamylation, which consists of t he sequential, posttranslational addition of one to six glutamyl units to both alpha- and beta-tubulin subunits, regulates the binding of Ta u as a function of its chain length. The relative affinity of Tau, ver y low for unmodified tubulin, increases progressively for isotubulins carrying from one to three glutamyl units, reaches an optimal value, a nd then decreases progressively when the polygutamyl chain lengthens u p to six residues. Our results suggest that the unmodified C-terminus of tubulin exerts a constitutive inhibition on Tau binding, probably b y locking the MAP-binding site, and that this inhibition could be firs t released and then restored as the polyglutamyl chain grows. As the p osttranslational chain does not appear to interact directly with Tau, it is thought that the growth of this chain from one to six glutamyl u nits causes a progressive, conformational shift in the structure of th e C-terminal domain of tubulin, thus leading to the observed modulatio n of affinity.