Cells support their fragile membranes with tough fibers that connect through their cytoplasmic membranes to adjacent cells. The unique overlapping and twisted conformation of the protein molecules in intermediate filaments makes them resistant to stretching (tensile strength). The meshes formed of intermediate filaments are usually stable, but they can be disassembled, e.g. during mitosis, when movement of chromosomes through the cytoplasm is essential.
Intermediate Filaments are Strong (as Nails)
These fibers are strong because of their structures. The proteins that make up the fibers are extended to their full length and twisted in pairs into overlapping tetramers. The tetramers lap together into long strands that are in turn twisted together eight at a time to make the intermediate filaments. Accessory proteins bind and crosslink intermediate filaments. Some groups of cells can accumulate large amounts of intermediate filament proteins, e.g. keratin, and this is the source of nails and hair.
Intermediate Filaments are Stable (as the Nucleus)
The mesh-like structures of intermediate filaments retain their forms, unlike the other cytoskeletal fibers, microtubules and actin microfilaments, that are inherently unstable. The nucleus, for example, has two lipid bilayers, the inner and outer nuclear membranes. The inner membrane is lined with a mesh of intermediate filaments, the nuclear lamina. Accessory proteins attach to the nuclear lamina and pass through both nuclear membranes to attach to motor that move along microtubules and keep the nucleus at the center of the cell.
Intermediate Filaments Can be Disassembled (for Mitosis)
The stability of the networks of intermediate filaments are required to maintain the shape of a cell, but they also make it impossible for large structures, such as chromosomes, to move through the cytoplasm. During cell division, mitosis, the proteins of the intermediate filaments are phosphorylated by an enzyme, protein kinase that controls the cell cycle. The phosphorylated proteins disassemble and permit chromosomes to move to each end of the cell during mitosis. Following mitosis, a nucleus is reassembled around the nuclear lamina and chromosomes of each daughter cell.
Intermediate Filaments are Essential (as part of the Cytoskeleton)
The cytoskeleton is composed of microtubules, intermediate filaments and actin microfilaments. Both the microtubules and actin microfilament are dynamic; they are constantly made and disassembled unless they are specifically modified or capped to retain their structure. The intermediate filaments are much more constant and are only disassembled prior to the formation of new cells, i.e. mitosis or meiosis. Thus, the intermediate filaments are neither the largest or the smallest cytoskeletal fibers, but they are strong and constant.
Reference
Bruce Alberts, The Molecular Biology of the Cell, 5th ed. (2008), Garland Science.
Join the Conversation