The 10-nm filaments of baby hamster kidney (BHK-21) cells, when examined either in the form of native ilanent caps or polymerized in vitro, are long tubes of protein 8-10 nm in diameter. They contain about 42% a-helix, which, on the basis of x-ray diffraction data, is arranged n a coiled-coil conformation characteristic of proteins of the a type. The known structural properties such as morphology, dimensions, subunit composition, and ultrastructure of this fibrous protein are very similar to those of the mammalian epidermal keratin filament, to which it may therefore be related. It is now apparent that all eukaryotic cell types contain at least three different types of intracellular structural proteins: microtubules (about 25 nm in diameter), actin-containing microfilaments (about 5-7 nm in diameter), and a less-characterized class of filaments of intermediate dimensions (7)(8)(9)(10)(11)(12) (6,14,15,21,22), and cell locomotion in cultured cells (2,17,18). In more specialized cells such as keratinocytes, these filaments comprise up to 70% of the total cellular mass (19,23) and, through interconnections between desmosomes, appear to lend a rigid or flexible texture to the tissue (19,24).The grouping of 10-nm filaments, neurofilaments, tonofilaments and other intermediate filaments into a class of similar fibrous proteins has been based almost exclusively on similarities in morphology and amino acid compositions of major subunits (4,17). Therefore, it is of considerable importance to use more quantitative chemical and structural techniques to determine the extent of homology within this class from different cell types. In the cells studied to date, the reported numbers of filament subunits vary from one to a large number, and their molecular weights are within the range 45,000-212,000 (4,5). However, it is unclear whether these differences are real, as suggested by peptide mapping (25), or artifacts due to peptide degradation (13).While comparatively little is known of the structure of this class of filaments from most cell types, knowledge of the structure of keratin filaments from both hair (and wool) and epidermis is now well advanced (26,27). Thus consideration and comparison of the properties-of keratin filaments with those of the similar-sized filaments from other cell types may be useful. Keratin filaments are a-helix-rich fibrous proteins that exhibit an a type x-ray diffraction pattern characteristic of proteins of the k ("hard" keratin)-m (myosin)-e (epidermin = epidermal keratin)-f (fibrin) class (19,28). This x-ray diffraction pattern has been interpreted in terms of models in which a-helical regions of the subunits are arranged in a twoor three-strandedl supercoiled or coiled-coil conformation (29,30). Direct evidence for this concept stems from the more recent characterization of a-helix-rich regions containing two (31, 32) or three coiled-coil chains (27,33,34) (X240,000). (C) Transverse cross-section through a fiber used for x-ray diffraction. (X240,000.) In A-C, the bar is 0. 1.um.(D) ...