Evidence for an Ordering Transition near 120 K in an Intrinsically Disordered Protein, Casein

Abstract

Intrinsically disordered proteins (IDPs) are proteins that possess large unstructured regions. Their importance is increasingly recognized in biology but their characterization remains a challenging task. We employed field swept Electron Spin Echoes in pulsed EPR to investigate low-temperature stochastic molecular librations in a spin-labeled IDP, casein (the main protein of milk). For comparison, a spin-labeled globular protein, hen egg white lysozyme, is also investigated. For casein these motions were found to start at 100 K while for lysozyme only above 130 K, which was ascribed to a denser and more ordered molecular packing in lysozyme. However, above 120 K, the motions in casein were found to depend on temperature much slower than those in lysozyme. This abrupt change in casein was assigned to an ordering transition in which peptide residues rearrange making the molecular packing more rigid and/or more cohesive. The found features of molecular motions in these two proteins turned out to be very similar to those known for gel-phase lipid bilayers composed of conformationally ordered and conformationally disordered lipids. This analogy with a simpler molecular system may appear helpful for elucidation properties of molecular packing in IDPs.

Structural properties of supercooled deep eutectic solvents: choline chloride–thiourea compared to reline

“…Finally, it is interesting to note also that for reline the Δ W temperature dependence coincides with that found previously for gel-phase phospholipid bilayers composed of 1,2-dipalmitoyl- sn -glycero-3-phosphocholine (DPPC) with incorporated spin-labeled 5-doxyl-stearic-acid 39 (which in turn is close to the Δ W temperature dependence in globular protein lysozyme 45 ); these data are also given in Fig. 4.…”

“…Except for the mentioned above ILs, nanoscale molecular packing was studied with pulsed EPR for model biological membranes (lipid bilayers) 37–42 and for proteins. 34,43–45…”

Librational Dynamics of Spin-Labeled Membranes at Cryogenic Temperatures From Echo-Detected ED-EPR Spectra

“…The root-mean-square angular amplitudes in unsaturated bilayers are among the highest obtained. Recently, it has been evidenced by pulse-EPR that the high mobility of unsaturated bilayers is comparable to that of regions of intrinsically disordered proteins ( Maslennikova et al, 2021 ).…”