Structure and Function of the Peripheral Membrane Protein P2 from Human Nervous System Myelin

The myelin sheath is a multilayered, tightly packed membrane wrapped around axons, enabling the rapid saltatory transmission of nerve impulses. Myelin carries a set of specific proteins, most of which are either integral or peripheral membrane proteins. In the peripheral nervous system, one of the myelin-specific proteins is P2, a member of the fatty acid binding protein (FABP) family that binds monomeric lipids inside its barrel structure. P2 is a peripheral membrane protein, and in addition to binding membrane surfaces, it is able to stack lipid bilayers. We performed a detailed structure-function analysis of recombinant human peripheral nerve P2. The liganded crystal structure of the wild-type human P2 protein was determined at 0.93-A resolution, allowing a very detailed analysis of fatty acid binding. In addition, 14 structure-based mutants were produced and characterized with respect to crystal and solution structure as well as membrane interactions and lipid binding. While the surface charge distribution of P2 presents two positively charged faces as putative membrane binding sites, the mutations with largest effects on P2 function did not affect the basic surface residues. Instead, it is likely they affect the dynamics and structure of the helical lid domain, which is expected to open and close upon lipid binding and to partially insert into the membrane. Furthermore, a hinge domain mutant could be crystallized in the absence of bound lipid. We also show that P2 gets oriented when binding to membrane surfaces. Overall, our current data allow a very detailed understanding of the structure-function relationships in myelin P2, a unique FABP, which is able to promote membrane multilayer formation.