Glutamate provides a key structural contact between reticulon-4 (Nogo-66) and phosphocholine

Journal Article

Published in: http://www.sciencedirect.com/science/article/pii/S000527361400193X

Human reticulon 4 (RTN-4) has been identified as the neurite outgrowth inhibitor (Nogo). This protein contains a span of 66 amino acids (Nogo-66) flanked by two membrane helices at the C-terminus. We previously determined the NMR structure of Nogo-66 in a native-like environment and defined the regions of Nogo-66 expected to be membrane em- bedded. We hypothesize that aromatic groups and a negative charge hyperconserved among RTNs (Glu26) drive the remarkably strong association of Nogo-66 with a phosphocholine surface. Glu26 is an isolated charge with no counter- ion provided by nearby protein groups. We modeled the docking of dodecylphosphocholine (DPC) with Nogo-66 and found that a lipid choline group could form a stable salt bridge with Glu26 and serve as a membrane anchor point. To test the role of the Glu26 anion in binding choline, we mutated this residue to alanine and assessed the structural con- sequences, the association with lipid and the affinity for the Nogo receptor. In an aqueous environment, Nogo-66 Glu26Ala is more helical than WT and binds the Nogo receptor with higher affinity. Thus, we can conclude that in the absence of a neutralizing positive charge provided by lipid, the glutamate anion is destabilizing to the Nogo-66 fold. Although the Nogo-66 Glu26Ala free energy of transfer from water into lipid is similar to that of WT, NMR data reveal a dramatic loss of tertiary structure for the mutant in DPC micelles. These data show that Glu26 has a key role in defining the structure of Nogo-66 on a phosphocholine surface.