Actin filaments structural features

The most detailed Z-band structure to date has come from the extended Z-crystals found in muscles of patients with nemaline myopathy (Morris et al., 1990). This showed the unit cell of the structure to have the symmetry 432i2i in which the actin filaments themselves have 43 screw symmetry (i.e., they are on a left-handed 4/1 helix). In an earlier section the helical symmetry of typical actin filaments was described as a 13/6 helix of repeat 357.5 A. However, this is only one of a family of closely related symmetries. The actin filaments in insect flight muscle have 28/13 symmetry (28 actin subunits in 13 turns) and a repeat of 770 A. The normal 13/6 helix could also be called a 26/12 helix (with a repeat of 715 A), which shows the similarity between the two structures one is a slightly unwound version of the other. One of the features of the 28/13 helix is that 28 is divisible by 4, which means that, starting from a given actin monomer, there must be three other monomers with exactly 90° azimuthal... 

All eukaryotes possess a cytoskeleton, and parasitic protozoa are no exception. Cytoskeletal structures, particularly microtubular elements, classically have been among the major features used in defining the taxonomic relationships of protozoa. The cytoskeleton of higher eukaryotes contains three major types of protein filaments intermediate filaments, actin filaments and microtubules. Intermediate filaments are composed of fibrous, overlapping proteins those found in mammalian cells (e.g. keratins and lamins) have not yet been studied in protozoa. 

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