Investigating the conformational coupling between the transmembrane and cytoplasmic domains of a single-spanning membrane protein. A 1H-NMR study

Membrane protein fragments reveal both secondary and tertiary structure of membrane proteins

Structural data on membrane proteins, while crucial to understanding cellular function, are scarce due to difficulties in applying to membrane proteins the common techniques of structural biology. Fragments of membrane proteins have been shown to reflect, in many cases, the secondary structure of the parent protein with fidelity and are more amenable to study. This chapter provides many examples of how the study of membrane protein fragments has provided new insight into the structure of the parent membrane protein.

Single-spanning membrane protein insertion in membrane mimetic systems: role and localization of aromatic residues

Membrane protein insertion in the lipid bilayer is determining for their activity and is governed by various factors such as specific sequence motifs or key amino-acids. A detailed fluorescence study of such factors is exemplified with PMP1, a small (38 residues) single-membrane span protein that regulates the plasma membrane H(+)-ATPase in yeast and specifically interacts with phosphatidylserines. Such interactions may stabilize raft domains that have been shown to contain H(+)-ATPase. Previous NMR studies of various fragments have focused on the critical role of interfacial residues in the PMP1 structure and intermolecular interactions. The C-terminal domain contains a terminal Phe (F38), a single Trp (W28) and a single Tyr (Y25) that may act together to anchor the protein in the membrane. In order to describe the location and dynamics of W28 and the influence of Y25 on protein insertion within membrane, we carried out a detailed steady-state and time-resolved fluorescence study of the synthetic G13-F38 fragment and its Tyr-less mutant, Y25L in various membrane mimetic systems. Detergent micelles are conveniently used for this purpose. We used dodecylphosphocholine (DPC) in order to compare with and complement previous NMR results. In addition, dodecylmaltoside (DM) was used so that we could apply our recently described new quenching method by two brominated analogs of DM (de Foresta et al. 2002, Eur. Biophys. J. 31:185-97). In both systems, and in the presence and absence of Y25, W28 was shown to be located below but close to the polar headgroup region, as shown by its maximum emission wavelengths (lambda(max)), curves for the quenching of Trp by the brominated analogs of DM and bimolecular constants for quenching (k(q)) by acrylamide. Results were interpreted by comparison with calibration data obtained with fluorescent model peptides. Time-resolved anisotropy measurements were consistent with PMP1 fragment immobilization within peptide-detergent complexes. We tentatively assigned the two major Trp lifetimes to the Trp (chi(1)=60 degrees and 180 degrees ) rotamers, based on the recent lifetime-rotamer correlation proposed for model cyclic peptides (Pan and Barkley 2004, Biophys J 86:3828-35). We also analyzed the role of the hydrophobic anchor, by comparing the micelle binding of fragments of various lengths including the synthesized full-length protein and detected peculiar differences for protein interaction with the polar headgroups of DM or DPC.

Deciphering the role of individual acyl chains in the interaction network between phosphatidylserines and a single-spanning membrane protein

PMP1 is a small single-spanning membrane protein functioning as a regulatory subunit of the yeast plasma membrane H(+)-ATPase. This protein forms a unique helix and exhibits a positively charged cytoplasmic domain that is able to specifically segregate phosphatidylserines (PSs). A marked groove formed at the helix surface is thought to play a major role in the related lipid-protein interaction network. Mutational analysis and (1)H NMR experiments were therefore performed on a synthetic PMP1 fragment using DPC-d(38) micelles as a membrane-like environment, in the presence of small amounts of POPS. A mutation designed for altering the helix groove was shown to disfavor the POPS binding specificity as much as that affecting the electrostatic interaction network. From POPS titration experiments monitored by a full set of one- and two-dimensional NOESY spectra, the association between the phospholipids and the PMP1 peptide has been followed. Our data reveal that the clustering of POPS molecules is promoted from a stabilized framework obtained by coupling the PMP1 helix groove to a POPS sn-2 chain. To our knowledge, the NOE-based titration plots displayed in this report constitute the first NMR data that directly distinguish the role of the sn-1 and sn-2 acyl chains in a lipid-protein interaction. The results are discussed while taking into account our accurate knowledge of the yeast plasma membrane composition and its ability to form functional lipid rafts.