STABLE, DETYROSINATED MICROTUBULES FUNCTION TO LOCALIZE VIMENTIN INTERMEDIATE FILAMENTS IN FIBROBLASTS

Separate populations of microtubules (MTs) distinguishable by their level of posttranslationally modified tubulin subunits and by their stability in vivo have been described. In polarized 3T3 cells at the edge of an in vitro wound, we have found a striking preferential coalignment of vimentin intermediate filaments (Ifs) with detyrosinated MTs (Glu MTs) rather than with the bulk of the MTs, which were tyrosinated MTs (Tyr MTs). Vimentin Ifs were not stabilizing the Glu MTs since collapse of the IF network to a perinuclear location, induced by microinjection of monoclonal anti-IF antibody, had no noticeable effect on the array of Glu MTs. To test whether Glu MTs may affect the organization of Ifs we regrew MTs in cells that had been treated with nocodazole to depolymerize all the MTs and to collapse Ifs; the reextension of Ifs into the lamella lagged behind the rapid regrowth of Tyr MTs, but was correlated with the slower reformation of Glu MTs. Similar realignment of Ifs with newly formed Glu MTs was observed in serum-starved cells treated with either serum or taxol to induce the formation of Glu MTs. Next, we microinjected affinity purified antibodies specific for Glu tubulin (polyclonal SG and monoclonal 4B8) and specific for Tyr tubulin (polyclonal W-2 and monoclonal YL1/2) into 3T3 cells. Both injected SG and 4B8 antibodies labeled the subset of endogenous Glu MTs; W-2 and YL1/2 antibodies labeled virtually all of the cytoplasmic MTs. Injection of SG or 4B8 resulted in the collapse of Ifs to a perinuclear region. This collapse was comparable to that observed after complete MT depolymerization by nocodazole. Injection of W-2, YL1/2, or nonspecific control IgGs did not result in collapse of the Ifs. Taken together, these results show that Glu MTs localize IFs in migrating 3T3 fibroblasts and suggest that detyrosination of tubulin acts as a signal for the recruitment of vimentin Ifs to MTs.