VIMENTINS TAIL INTERACTS WITH ACTIN-CONTAINING STRUCTURES IN-VIVO

The tail domain of the intermediate filament (IF) protein vimentin is unnecessary for IF assembly in vitro. To study the role of vimentin's tail in vivo, we constructed a plasmid that directs the synthesis of a 'myc-tagged' version of the Xenopus vimentin-l tail domain in bacteri a. This polypeptide, mycVimTail, was purified to near homogeneity and injected into cultured Xenopus A6 cells. In these cells the tail polyp eptide co-localized with actin even in the presence of cytochalasin. T wo myc-tagged control polypeptides argue for the specificity of this i nteraction. First, a similarly myc-tagged lamin tail domain localizes to the nucleus, indicating that the presence of the myc tag did not it self confer the ability to co-localize with actin (Hennekes and Nigg ( 1994) J. Cell Sci. 107, 1019-1029). Second, a myc-tagged polypeptide w ith a molecular mass and net charge at physiological pH (i.e. -4) simi lar to that of the mycVimTail polypeptide, failed to show any tendency to associate with actin-containing structures, indicating that the in teraction between mycVimTail and actin-containing structures was not d ue to a simple ionic association. Franke (1987; Cell Biol. Int. Rep. 1 1, 831) noted a similarity in the primary sequence between the tail of the type I keratin DG81A and vimentin. To test whether the DG81A tail interacted with actin-containing structures, we constructed and purif ied myc-tagged DG81A tail polypeptides. Unexpectedly, these keratin ta il polypeptides were largely insoluble under physiological conditions and formed aggregates at the site of injection. While this insolubilit y made it difficult to determine if they associated with actin-contain ing structures, it does provide direct evidence that the tails of vime ntin and DG81A differ dramatically in their physical properties. Our d ata suggest that vimentin's tail domain has a highly extended structur e, binds to actin-containing structures and may mediate the interactio n between vimentin filaments and microfilaments involved in the contro l of vimentin filament organization (Hollenbeck et al. (1989) J. Cell Sci. 92, 621; Tint et al. (1991) J. Cell Sci. 98, 375).