Posttranslational Modification of Intermediate Filament Proteins

Abstract

Intermediate filaments have been recognized as a class of cell organelles which, in contrast to the other cellular filament systems, exhibit the strong tendency to retain their polymerized state and cytoplasmic distribution. However, their occasionally dramatic reorganization, for example, during mitosis (see Sect. 2.3.1) or in response to treatment of cells with drugs (see Sect. 2.3.5), shows that they are even less inert than it might appear at first. There are indications that the reactivity of intermediate filaments is regulated by posttranslational modification of their subunit proteins. The most frequently observed type of posttranslational modification of proteins is phosphorylation (Alix and Hayes 1983), and, indeed, all intermediate filament proteins have been found to occur to a low extent in their phosphorylated forms. Moreover, intermediate filaments and their subunit proteins are highly susceptible to limited degradation by Ca²⁺-activated proteinases. It is certainly a rewarding problem to investigate whether the reactivity of intermediate filaments is influenced by partial, Ca²⁺-dependent proteolysis and whether their proteolytically processed subunit proteins are charged with specific cellular functions. Since Ca²⁺-activated proteinases might play an important role in the functioning of intermediate filaments and their subunit proteins, a considerable part of this section will be devoted to the description of their functional properties and distribution in vertebrate cells.