**Jocelyne Walter \*, Amandine Barra, Bénédicte Doublet, Nicolas Céré, Justine Charon † and Thierry Michon \***

UMR 1332 Biologie du Fruit et Pathologie, INRA, Université de Bordeaux, CS 20032, 33140 Villenave d'Ornon, France; amandine.barra@inra.fr (A.B.); benedicte.doublet@inra.fr (B.D.); nicolas.cere@hotmail.fr (N.C.); justine.charon@sydney.edu.au (J.C.)

**\*** Correspondence: Jocelyne.walter@inra.fr (J.W.); thierry.michon@inra.fr (T.M.)

† Present address: School of Life & Environmental Sciences, University of Sydney, Sydney, NSW 2006, Australia.

Received: 13 March 2019; Accepted: 5 April 2019; Published: 11 April 2019

**Abstract:** Protein intrinsic disorder is involved in many biological processes and good experimental models are valuable to investigate its functions. The potyvirus genome-linked protein, VPg, displays many features of an intrinsically disordered protein. The virus cycle requires the formation of a complex between VPg and eIF4E, one of the host translation initiation factors. An in-depth characterization of the hydrodynamic properties of VPg, eIF4E, and of their binary complex VPg-eIF4E was carried out. Two complementary experimental approaches, size-exclusion chromatography and fluorescence anisotropy, which is more resolving and revealed especially suitable when protein concentration is the limiting factor, allowed to estimate monomers compaction upon complex formation. VPg possesses a high degree of hydration which is in agreement with its classification as a partially folded protein in between a molten and pre-molten globule. The natively disordered first 46 amino acids of eIF4E contribute to modulate the protein hydrodynamic properties. The addition of an N-ter His tag decreased the conformational entropy of this intrinsically disordered region. A comparative study between the two tagged and untagged proteins revealed the His tag contribution to proteins hydrodynamic behavior.

**Keywords:** intrinsically disordered protein; plant virus; eIF4E; VPg; potyvirus; molten globule; protein-protein interaction; fluorescence anisotropy; protein hydrodynamics
