Structure and dynamics of a human myelin protein P2 portal region mutant indicate opening of the \u03b2 barrel in fatty acid binding proteins

Saara Laulumaa,Anushik Safaryan,Oda C Krokengen,Guillaume Brysbaert,Arne Raasakka,Petri Kursula,Salla Ruskamo,Tuomo Nieminen,Erik I Hallin,Marc F Lensink,Ilpo Vattulainen

doi:10.1186/s12900-018-0087-2

Abstract

BackgroundMyelin is a multilayered proteolipid sheath wrapped around selected axons in the nervous system. Its constituent proteins play major roles in forming of the highly regular membrane structure. P2 is a myelin-specific protein of the fatty acid binding protein (FABP) superfamily, which is able to stack lipid bilayers together, and it is a target for mutations in the human inherited neuropathy Charcot-Marie-Tooth disease. A conserved residue that has been proposed to participate in membrane and fatty acid binding and conformational changes in FABPs is Phe57. This residue is thought to be a gatekeeper for the opening of the portal region upon ligand entry and egress.ResultsWe performed a structural characterization of the F57A mutant of human P2. The mutant protein was crystallized in three crystal forms, all of which showed changes in the portal region and helix α2. In addition, the behaviour of the mutant protein upon lipid bilayer binding suggested more unfolding than previously observed for wild-type P2. On the other hand, membrane binding rendered F57A heat-stable, similarly to wild-type P2. Atomistic molecular dynamics simulations showed opening of the side of the discontinuous β barrel, giving important indications on the mechanism of portal region opening and ligand entry into FABPs. The results suggest a central role for Phe57 in regulating the opening of the portal region in human P2 and other FABPs, and the F57A mutation disturbs dynamic cross-correlation networks in the portal region of P2.ConclusionsOverall, the F57A variant presents similar properties to the P2 patient mutations recently linked to Charcot-Marie-Tooth disease. Our results identify Phe57 as a residue regulating conformational changes that may accompany membrane surface binding and ligand exchange in P2 and other FABPs.

Highlights

Myelin is a multilayered proteolipid sheath wrapped around selected axons in the nervous system
Myelin P2 folding and stability The F57A and P38G mutants were compared to Wild-type P2 (wt-P2) using circular dichroism (CD) spectroscopy (Fig. 1a)
A large effect on the CD spectra of membrane-bound P2 was induced by both mutations (Fig. 1b), and difference spectra indicate that the mutant variants contain less overall secondary structure than wt-P2

Summary

Introduction

Myelin is a multilayered proteolipid sheath wrapped around selected axons in the nervous system. P2 is a myelin-specific protein of the fatty acid binding protein (FABP) superfamily, which is able to stack lipid bilayers together, and it is a target for mutations in the human inherited neuropathy Charcot-Marie-Tooth disease. A multitude of proteins interact with lipid membrane surfaces These peripheral membrane proteins come from various protein families, and have generally little in common at the structural level. Three point mutations in human P2 were linked to Charcot-Marie-Tooth disease (CMT), an inherited neuropathy [8,9,10] We showed that these mutations all decrease the stability of P2, they did not lead to major differences in the P2 fold in the crystal state [11]. The disease mutations slightly altered membrane binding properties of P2 [11]

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Conclusion