Actin is composed of four domains with a large cleft almost bisecting the molecule. This nomenclature arises from the resemblance of microfilaments decorated with fragments of myosin II to arrowheads in the electron microscope. The microfilament is a single-stranded helix with each monomer rotated 166 o with respect to neighbouring subunits which means that every 36 nm, or every 13 subunits, subunits eclipse each other at what appears to be a crossover. As their name suggests MTs are small tubes. They are 25nm in diameter with an internal diameter of 14nm.
IFs are different to microfilaments and microtubules in a number of fundamental respects.
IF proteins are almost the only protein. In other cell types, IFs are modified by phosphorylation when they are required to be disassembled for example during cell division. Unlike the highly conserved actins and tubulins more than 40 distinct IF proteins are encoded by a number of genes in mammalian cells. All IF proteins have a similar structure with a central helical rod domain and more variable head and tail domains. Basic Keratins Epithelia iii Desmin Muscle Glial Glial cells and astrocytes Peripherin Peripheral neurones Vimentin Mesenchyme iv Neurofilaments Neurons v Lamins Nuclear envelopes Figure 4.