To ensure a nonbiased evaluation of search hits, (E 4K 4) 2 and (E 4R 4) 2 were used as search templates, and the list of sequences that scored better than any species of myosin X or myosin VI was examined to assess the relative number of proteins that contain the ER/K motif. The number of both known and hypothetical or predicted protein sequences in the database with the ER/K motif is revealed by a BLAST search in the PubMed database of nonredundant protein sequences including all species. We propose that the significant rigidity of the ER/K α-helix can help regulate protein function, as a force transducer between protein subdomains. Thus, the ER/K α-helix is an isolated secondary structural element that can efficiently span long distances in proteins, making it a promising tool in designing synthetic proteins. Results of small-angle x-ray scattering (SAXS) and single-molecule optical-trap analyses are consistent with the high bending rigidity predicted by our model. A simplified model predicts that side-chain interactions alone contribute substantial bending rigidity (0.5 pN/nm) to ER/K α-helices. By using molecular dynamics (MD) simulations, we characterize a dynamic pattern of side-chain interactions that extends along the backbone of ER/K α-helices. Here, we find that this ER/K motif is more prevalent than previously reported, being represented in proteins of diverse function from archaea to humans. However, amino acid sequences in proteins that are based on alternating repeats of four glutamic acid (E) residues and four positively charged residues, a combination of arginine (R) and lysine (K), have been shown to form stable α-helices in a few proteins, in the absence of tertiary interactions. Protein α-helices are ubiquitous secondary structural elements, seldom considered to be stable without tertiary contacts.
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