IF Proteins Are Classified According to Their Distributions in Specific Tissues

In higher vertebrates, the subunits composing intermediate filaments constitute a superfamily of highly a helical proteins that are found in the cytoplasm of different tissues and at the nuclear membrane. The superfamily is divided into four groups on the basis of similarities in sequence and their patterns of expression in cells (Table 19-4). Unlike the actin and tubulin isoforms, the various classes of IF proteins are widely divergent in sequence and vary greatly in molecular weight. We introduce the four groups here and consider their functions in various cells in more detail later.

The most ubiquitous group of IFs are the lamins. In contrast with the cytosolic location of the other four classes of IF proteins, lamins are found exclusively in the nucleus. Of the three nuclear lamins, two are alternatively spliced products encoded by a common gene, whereas the third is encoded by a separate gene. A single lamin gene is found in the

TABLE 19-4 Primary Intermediate Filaments in Mammals

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