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

Natalya A Maslennikova,Elena A Golysheva,Sergei A Dzuba

doi:10.3390/molecules26195971

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.

Highlights

The protein functions are determined by their three-dimensional tertiary structures.there exist proteins possessing unstructured regions of significant size, being biologically active; these proteins are called intrinsically disordered proteins (IDPs), as reviewed in [1,2,3,4,5,6,7]
For TEMPOL in hydrated casein, the continuous wave (CW) electron paramagnetic resonance (EPR) spectrum shows a motional narrowing that is expected because of its hydrophilic nature and the location in the hydration layer
Additional information on the spin label location in proteins can be obtained in three-pulse stimulated electron spin echo envelope modulation (ESEEM) experiments on

Summary

Introduction

The protein functions are determined by their three-dimensional tertiary structures. There exist proteins possessing unstructured regions of significant size, being biologically active; these proteins are called intrinsically disordered proteins (IDPs), as reviewed in [1,2,3,4,5,6,7]. IDPs carry out various functional roles, such as signal transduction and storage of small molecules. IDPs recognize proteins, nucleic acids, and other types of binding molecules and accelerate interactions and biochemical reactions between bound partners; biological activities of IDPs complement those of structured proteins. Characterization of IDPs and intrinsically disordered regions (IDRs) in proteins can be done by different spectroscopic methods: nuclear magnetic resonance (NMR), small-angle. Electron paramagnetic resonance (EPR) spectroscopy has proven to be a valuable tool to study functional properties of IDPs [9,10,11,12]

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Conclusion