Intermediate filaments (IFs; diameter, about 10 nm) are cytoplasmic cytoskeletal structures found in most vertebrate cells. Their protein subunits comprise a large multi-gene family of related proteins, which make it possible to divide IFs into seven separate classes whose expression is cell-type-dependent. The most important IF classes are cytokeratin (CK) filaments (epithelial cells), vimentin filaments (mesenchymal cells), desmin filaments (muscle cells), glial filaments (astrocytes), and neurofilaments (nerve cells). As the specificity of expression of IF proteins is retained in malignant tumors, they are suitable as histological markers of differentiation (tumor markers). The protein subunits of the epithelial CK filaments are unusually diverse, and within the various types of epithelia, their expression is differentiation specific. Until recently, the catalog of human CKs comprised 19 related, yet distinct polypeptides (CKs 1-19; Moll et al., 1982a); CK 20 can now be added to this list. On the basis of sequence relationships, two CK subfamilies can be delineated (CKs 9-20 = type I; CKs 1-8 = type II). Any given epithelial cell exhibits a characteristic, differentiation-dependent combination of two or more CK polypeptides, with type-I and -II polypeptides always occurring in stoichiometric amounts (i.e., as "pairs"), because the basic structural unit of the CK filaments is a heterotypical tetramer complex. On the basis of their main tissue distribution patterns, it is possible further to subdivide these polypeptides into CKs typical of stratified squamous epithelia (CKs 1-6, 9-17) and those typical of simple columnar epithelia (CKs 7, 8, 18-20); these CKs exhibit differential expression patterns in the various types of squamous and columnar epithelia. The actual characterization of the novel CK 20 as a CK initially proved to be rather difficult, as this cytoskeletal protein, which can be biochemically isolated from cells of the intestinal epithelium (M(r) 46,000; previously called "IT protein"), exhibits no reaction with numerous well-known CK antibodies in Western blots. However, a series of other characteristics typical of CKs could be demonstrated. Thus, IT protein was found, in vitro (nitrocellulose-blot binding test, native gel electrophoresis), to for heterotypical complexes with the type-II CK 8, and these complexes were able to reconstitute themselves into typical IFs in vitro. Chymotrypsin-cleaving experiments revealed the presence of a resistant core fragment (M(r) 38,000), indicating a alpha-helical "coiled-coil" conformation typical of IFs.(ABSTRACT TRUNCATED AT 400 WORDS)