A relatively unknown protein structure motif forms stable isolated single α-helices, termed ER/K α-helices, in a wide variety of proteins and has been shown to be essential for the function of some molecular motors. The flexibility of the ER/K α-helix determines whether it behaves as a force transducer, rigid spacer, or flexible linker in proteins. In this study, we quantify this flexibility in terms of persistence length, namely the length scale over which it is rigid. We use single-molecule optical trapping and small-angle x-ray scattering, combined with Monte Carlo simulations to demonstrate that the Kelch ER/K α-helix behaves as a wormlike chain with a persistence length of ∼15 nm or ∼28 turns of α-helix. The ER/K α-helix length in proteins varies from 3 to 60 nm, with a median length of ∼5 nm. Knowledge of its persistence length enables us to define its function as a rigid spacer in a translation initiation factor, as a force transducer in the mechanoenzyme myosin VI, and as a flexible spacer in the Kelch-motif-containing protein.
Bibliographical noteFunding Information:
Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (contract No. DE-AC02-06CH11357). S.S. is supported by an American Cancer Society postdoctoral fellowship, M.A. by a Stanford University graduate fellowship, and S.D. and J.A.S. by grants PO1 GM066275 and GM33289, both from the National Institutes of Health.