High molecular weight polypeptides (270,000-340,000) from cultured cells are related to hog brain microtubule-associated proteins but copurify with intermediate filaments.

High molecular weight polypeptides (HMWPs) of 270,000 to 340,000 were found to be major components of intermediate filaments prepared by Triton X-100 extraction after spreading of rat glioma C6, HeLa, Chinese hamster ovary, and simian virus 40-transformed Chinese hamster lung cells. C6 HMWPs were shown to resemble high molecular weight microtubule-associated proteins from hog brain by four criteria: (i) comigration in electrophoresis on high-resolution sodium dodecyl sulfate/polyacrylamide gels, (ii) one-dimensional peptide mapping, (iii) phosphorylation in vitro with [gamma-32P]ATP, and (iv) ability to promote microtubule assembly in vitro. HMWPs were also found to be major components of one-time polymerized C6 microtubule preparations, which contained a sizable amount of intermediate filaments. The predominant part of HMWPs present in these microtubule preparations was found not to copurify with microtubules in cycles of temperature-dependent assembly/disassembly but to remain with the cold-insoluble intermediate filaments. These results provide an explanation for the low yields that have hampered attempts to purify microtubule-associated porteins, in particular HMWPs, from cultured cells in the past. Moreover, they suggest that HMWPs might have a dual role in the cell, serving not only as regulators of microtubule assembly but also as linker components between microtubules and intermediate filaments.