Atomic resolution view into the structure-function relationships of the human myelin peripheral membrane protein P2.

Salla Ruskamo,Satyan Sharma,Mikael Simons,Mari Lehtimäki,Inari Kursula,Petri Kursula,Shweta Aggarwal,Anne S Ulrich,Ravi P Yadav,Saara Laulumaa,Jochen Bürck,André H Juffer

doi:10.1107/s1399004713027910

Abstract

P2 is a fatty acid-binding protein expressed in vertebrate peripheral nerve myelin, where it may function in bilayer stacking and lipid transport. P2 binds to phospholipid membranes through its positively charged surface and a hydrophobic tip, and accommodates fatty acids inside its barrel structure. The structure of human P2 refined at the ultrahigh resolution of 0.93 Å allows detailed structural analyses, including the full organization of an internal hydrogen-bonding network. The orientation of the bound fatty-acid carboxyl group is linked to the protonation states of two coordinating arginine residues. An anion-binding site in the portal region is suggested to be relevant for membrane interactions and conformational changes. When bound to membrane multilayers, P2 has a preferred orientation and is stabilized, and the repeat distance indicates a single layer of P2 between membranes. Simulations show the formation of a double bilayer in the presence of P2, and in cultured cells wild-type P2 induces membrane-domain formation. Here, the most accurate structural and functional view to date on P2, a major component of peripheral nerve myelin, is presented, showing how it can interact with two membranes simultaneously while going through conformational changes at its portal region enabling ligand transfer.

Highlights

The myelin sheath is a unique domain of a glial cell plasma membrane which forms tightly packed, ordered and multilayered proteolipid structures that wrap around selected axons in the nervous system
The 16-carbon palmitate, which is very abundant in E. coli, is not the only fatty acid present in the crystal, as was concluded earlier based on the 1.8 Aresolution electron density (Majava et al, 2010); there is an additional partial occupancy of an 18-carbon fatty acid (Fig. 1b, Supplementary Fig. S21)
Recombinant protein 2 (P2) expressed in E. coli can form stable complexes with both saturated and non-saturated fatty acids from the expression host

Summary

Introduction

The myelin sheath is a unique domain of a glial cell plasma membrane which forms tightly packed, ordered and multilayered proteolipid structures that wrap around selected axons in the nervous system. Myelin contains very little solvent and carries a set of specific proteins that are present at high concentration and are thought to mediate myelin membrane stacking and stabilization. Myelin protein 2 (P2) is a small highly basic protein of the fatty acid-binding protein (FABP) family that is expressed in peripheral nerve myelin sheaths in high abundance (Trapp et al, 1984). The FABP family includes proteins involved in fatty-acid transport (Storch & Thumser, 2000). For most FABPs, a collisional transfer model of ligands to/from the membrane has either been suggested or detected (Storch & Thumser, 2000), involving a direct interaction of the protein with the lipid bilayer, as well as electrostatic and hydrophobic forces between the helical lid domain and the membrane (Storch & McDermott, 2009). The functional details of P2 in doi:10.1107/S1399004713027910 165 research papers

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