Date Published: April 3, 2017
Publisher: Public Library of Science
Author(s): Dominic Simm, Klas Hatje, Martin Kollmar, Maria Gasset.
Stable single-alpha helices (SAHs) are versatile structural elements in many prokaryotic and eukaryotic proteins acting as semi-flexible linkers and constant force springs. This way SAH-domains function as part of the lever of many different myosins. Canonical myosin levers consist of one or several IQ-motifs to which light chains such as calmodulin bind. SAH-domains provide flexibility in length and stiffness to the myosin levers, and may be particularly suited for myosins working in crowded cellular environments. Although the function of the SAH-domains in human class-6 and class-10 myosins has well been characterised, the distribution of the SAH-domain in all myosin subfamilies and across the eukaryotic tree of life remained elusive. Here, we analysed the largest available myosin sequence dataset consisting of 7919 manually annotated myosin sequences from 938 species representing all major eukaryotic branches using the SAH-prediction algorithm of Waggawagga, a recently developed tool for the identification of SAH-domains. With this approach we identified SAH-domains in more than one third of the supposed 79 myosin subfamilies. Depending on the myosin class, the presence of SAH-domains can range from a few to almost all class members indicating complex patterns of independent and taxon-specific SAH-domain gain and loss.
Helices, which are not buried within globular structures or coiled-coil helical dimers, usually need networks of charge interactions for stabilization in water [1–5]. In the late 1980th and early 1990th many studies have been performed using poly-alanine peptide models aiming to resolve the conditions for helix formation and stabilization. Different amino acids (mainly aspartic acid, glutamic acid, lysine and arginine, but in some cases also glutamine) were introduced into these peptides alone and in all possible combinations at varying distances. The corresponding peptides were synthesised, their α-helicity experimentally determined by, for example, circular dichroism, and stabilization energies were obtained by fitting models to the data. Although poly-alanine peptides adopt α-helical conformations when sparsely interrupted by lysine , arginine  and glutamine residues , helices are especially stabilized by charged interactions (salt bridges) between residues at (i, i+3) and (i, i+4) spacing [1–3] and hydrogen-bonding interactions between polar/charged residues at (i, i+3) and (i, i+4) [4,5]. Additional stability is obtained through networks of oppositely charged residues in (i, i+3, i+6), (i, i+3, i+7), (i, i+4, i+7), or (i, i+4, i+8) distances [9,10]. In addition to these poly-alanine based peptides, studies have been performed on peptides with complex amino acid distributions [11,12]. However, each study used different combinations of residues and non-physiological experimental conditions were applied (e.g. salt concentrations).