Platelets contain a 210K microtubule-associated protein related to a similar protein in HeLa cells

We have demonstrated the presence of a 210K (K = 10(3) Mr) microtubule-associated protein (MAP) in blood platelets and have studied its relationship to tubulin and to the cytoskeleton, using a well-characterized polyclonal antibody for the analysis. When platelet lysates were enriched for tubulin by an assembly cycle at 37 degrees C, the 210K MAP was also enriched, as detected by Western blotting, while the antigen was not detected in pellets from cold-treated samples that lacked stabilized tubulin. Immunofluorescence of resting platelets showed that the 210K antigen colocalized with the microtubule coil in ring-like structures. On the other hand, in preparations of platelet cytoskeletons, the 210K antigen was present in samples from platelets in which the coil was disassembled (cold-treated without taxol pretreatment) as well as from platelets in which the coil was preserved (at 37 degrees C without taxol, or 4 degrees C with taxol pretreatment). In chilled platelets with disassembled microtubule coils, indirect immunofluorescence using antibodies to 210K or tubulin gave a diffuse signal throughout the platelet cytoplasm. However, immunofluorescence of the 210K antigen in both resting and cold-treated platelets displayed discrete or patchy staining as compared to the continuous staining with antitubulin. We conclude that 210K MAP is present in platelets, that it copurifies with tubulin and that it is localized along the microtubule coil. Our results also suggest that the 210K MAP may interact with some other element(s) of the cytoskeleton, and hence that it might serve as a linking protein.