Intermediate filaments

Intermediate filaments form a class of insoluble cytoplasmic filaments ofdimensions intermediate in size between those of microtubules (about 25 nm in diameter) and microfilaments (about 7 nm in diameter). Thus, they are generally known as intermediate-sized filaments (IF)' or 10-nm filaments. They have been identified in virtually all differentiated types ofcells and it seems possible that IF-like proteins also are present in even the simplest eukaryote cell types. With immunological, biochemical, and molecular biological techniques, it has been possible to distinguish at least five distinct classes of proteins that constitute IF in different cells or tissues (30, 31, 46, 67): desmin or skeletin in myogenic cells; neurofilaments in neu-ronal cells; glial filaments in astroglia and related cells ; vimen-tin or decamin in mesenchymally derived cells; and a more complex group of keratins or cytokeratins in epithelial tissues. Some embryonic cells seem not to contain IF, but further studies are required to confirm this (36). Although this simple cell-type classification system is of considerable value in understanding IF and has provided information on cellular origins during development, there is a growing list of addenda and exceptions. For example, it was recognized some time ago that some cells express vimentin in addition to their cell-type class of IF subunit (32, 67). Some fibroblastic cell lines have been shown to possess keratin-like IF proteins associated with their nuclear surfaces (69). We expect that more exceptions will be identified in the future, possibly requiring a revised system ofIF subunit nomenclature. Each IF system is currently the subject of intense research and the understanding of IF is increasing rapidly. Here, we will briefly review some ofthe salient features ofthe different types of IF and present recent data from our laboratories on structural-functional aspects of IF. Keratin IF are present in large amounts in various epithelial tissues throughout higher organisms. They are insoluble in neutral aqueous solutions but can be extracted in the form of their constituents subunits in denaturing solvents such as sodium dodecyl sulfate or 8 M urea. Subsequent one-or two-dimensional gel electrophoresis oftissue extracts has identified 'Abbreviations used in this paper: 22s about 19 different subunits in human epithelial tissues, including specialized epithelia such as the epidermis, but when the keratin IF subunits of various epithelial derivatives (hair, nail, etc .) are included, the total number may approach 30 (35). Their molecular weights vary from 40,000 to 70,000. On two-dimensional gels, the subunits …