Intermediate Filament Networks: Organization and Possible Functions of a Diverse Group of Cytoskeletal Elements

Abstract

SUMMARY Immunofluorescence and electron microscopic observations demonstrate that intermediate filaments (IF) form cytoplasmic networks between the nucleus and cell surface in several types of cultured cells. Intermediate filaments interact with the nuclear surface, where they appear to terminate at the level of the nuclear envelope. From this region, they radiate towards the cell surface where they are closely associated with the plasma membrane. On the basis of these patterns of IF organization, we suggest that IF represent a cytoskeletal system interconnecting the cell surface with the nucleus. Furthermore, IF also appear to interact with other cytoskeletal components including microtubules and microfilaments. In the former case microtubule–IF interactions are seen in cytoplasmic regions between the nucleus and the cell membrane, whereas microfilament–IF interactions occur in the cortical cytoplasm. IF also appear to be cross-linked to each other; especially in the case of the IF bundles that occur in epithelial cells. In order to determine the molecular and biochemical bases of the organizational state of IF we have developed procedures for obtaining IF-enriched ‘cytoskeletons’ of cultured cells. In these preparations IF–nuclear and IF–cell surface associations are retained. Thus, these preparations have enabled us to begin to study various IF-associated structures (e.g. desmosomes) and associated proteins (IFAPs) using biochemical and immunological methodologies. To date, the results support the idea that IF and their associated proteins may comprise the cell type specific molecular infrastructure that is involved in transmitting and distributing information amongst the major cellular domains; the cell surface/extracellular matrix, the cytoplasm and the nuclear surface/nuclear matrix.