keratins of stratifed epithelia, and structural keratins, also called hard keratins. The latest are found in skin appendages such as hair and nails. Keratin filaments span the cytoplasm of epithelial cells and, via desmosomes, connect with keratin filaments in neighboring cells. Keratin subunits do not co-assemble with other classes of intermediate filaments; therefore, they form a distinct cell-specific and tissuespecific recognition system.
 Class 3. This group contains four proteins: vimentin, the most widely distributed intermediate filament protein in the body, and vimentin-like proteins such as desmin, glial fbrillary acidic protein (GFAP), and peripherin. They represent a diverse family of cytoplasmic filaments found in many cell types. In contrast to keratins, class 3 proteins (with the exception of desmin) preferentially form homopolymeric filaments containing only one type of intermediate protein. Vimentin is the most abundant intermediate filament found in all mesoderm-derived cells, including fibroblasts (Fig. 2.50); desmin is characteristic of muscle cells; GFAP is found in glial cells (highly specific for astrocytes), and peripherin is found in many peripheral nerve cells.
 Class 4. Historically this group has been called neuroflaments; they contain intermediate filament proteins that are expressed mostly in axons of nerve cells. The three types of neurofilament proteins are of different molecular weights: NF-L (a low-weight protein), NF-M (a medium-weight protein), and NF-H (a high-weight protein). They co-assemble to form a heterodimer that contains one NF-L molecule and one of the others. All three proteins form neurofilaments that extend from the cell body into the ends of axons and dendrites, providing structural support. However, genes for class 4 proteins also encode several other intermediate filament proteins. These include nestin and -internexin in nerve cells as well as synemin, syncoilin, and paranemin in muscle cells. Members of this group preferentially co-assemble in tissues as heteropolymers.
 Class 5. Lamins, specifically nuclear lamins, form a network-like structure that is associated with the nuclear envelope. Lamins are represented by two types of proteins, lamin A and lamin B. In contrast to other types of intermediate filaments found in the cytoplasm, lamins are located within the nucleoplasm of almost all differentiated cells in the body. A description of their structure and function can be found on page 82.
 Class 6. This is a lens-specific group of intermediate filament, or “beaded flaments” containing two proteins, phakinin and flensin. The periodic beadlike surface appearance of these filaments is attributed to the globular structure of the carboxy-terminus of the filensin molecule, which projects out from the assembled filament core.
Intermediate filament–associated proteins are essential for the integrity of cell-to-cell and cell-to-extracellular matrix junctions.
A variety of intermediate flament–associated proteins function within the cytoskeleton as integral parts of the molecular architecture of cells. Some proteins, such as those of the plectin family, possess binding sites for actin filaments,