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Intermediate filaments are found in almost all animals, but are not present in plant or fungi cells. These rope-like structures act as a scaffold for proteins to latch onto when trafficking them through the cell, as well as are important for structure and positioning.

One thing to particularly note about intermediate filaments is their importance in the nuclear lamina of the cell’s nucleus, where not only do they help maintain the shape of the nucleus/nuclear lamina, but also assist in transcriptional regulation (as they bind and act as a base for transcription factors), nuclear pore positioning and function, and heterochromatin organization.

An example of the importance of intermediate filaments in the cell is looking into case studies of those who suffer from Hutchinson-Gilford Progeria Syndrome. This disease results from a mutation in the coding sequence of the Lamin A intermediate filament protein, resulting in altered/irregular nuclei leading to premature aging in patients.

Microtubules are thick, stiff tube-like structures that are comprised of tubulin subunits. These filaments are stable and short-lived structures that polymerize when needed, for example when forming the spindles during the process of mitosis. These structures regularly oscillate between shortening and growing phases, enabling quick assembly and disassembly. The process of polymerizing microtubules uses GTP, and are built from alpha and beta tubulin building blocks. The elongation of microtubules also occurs on the + ends.

However, microtubules are also important for trafficking along the secretory pathway. A study done by J S Presley et al. visualizes the importance of microtubules in the cell, as cells treated with the drug nocodazole fail to migrate through the ER-golgi transport pathway, as nocodazole prevents disassembly of microtubules.

Finally, actin filaments are well, filaments made of actin. Actin exists in a globular form known as G-actin, and is one of the most abundant protein in cells. Each G-actin monomer can polymerize into a filament, which is referred to as F-actin. This elongation and assembly process of actin requires ATP. Unlike microtubules, actin filaments can grow or shrink on either the + or - ends of the filament (however the process can be halted by capping the ends with a capping protein). However the polymerization process is also regulated by proteins that assist and stimulate the assembly (ex. ARP2).