Alternative packing of EGFR transmembrane domain suggests that protein-lipid interactions underlie signal conduction across membrane.

The human epidermal growth factor receptor (EGFR) of HER/ErbB receptor tyrosine kinase family mediates a broad spectrum of cellular responses transducing biochemical signals via lateral dimerization in plasma membrane, while inactive receptors can exist in both monomeric and dimeric forms. Recently, the dimeric conformation of the helical single-span transmembrane domains of HER/ErbB employing the relatively polar N-terminal motifs in a fashion permitting proper kinase activation was experimentally determined. Here we describe the EGFR transmembrane domain dimerization via an alternative weakly polar C-terminal motif A(661)xxxG(665) presumably corresponding to the inactive receptor state. During association, the EGFR transmembrane helices undergo a structural adjustment with adaptation of inter-molecular polar and hydrophobic interactions depending upon the surrounding membrane properties that directly affect the transmembrane helix packing. This might imply that signal transduction through membrane and allosteric regulation are inclusively mediated by coupled protein-protein and protein-lipid interactions, elucidating paradoxically loose linkage between ligand binding and kinase activation.

Impact of membrane partitioning on the spatial structure of an S-type cobra cytotoxin.

Cobra cytotoxins (CTs) belong to the three-fingered protein family. They are classified into S- and P-types, the latter exhibiting higher membrane-perturbing capacity. In this work, we investigated the interaction of CTs with phospholipid bilayers, using coarse-grained (CG) and full-atom (FA) molecular dynamics (MD). The object of this work is a CT of an S-type, cytotoxin I (CT1) from N.oxiana venom. Its spatial structure in aqueous solution and in the micelles of dodecylphosphocholine (DPC) were determined by 1H-NMR spectroscopy. Then, via CG- and FA MD-computations, we evaluated partitioning of CT1 molecule into palmitoyloleoylphosphatidylcholine (POPC) membrane, using the toxin spatial models, obtained either in aqueous solution, or detergent micelle. The latter model exhibits minimal structural changes upon partitioning into the membrane, while the former deviates from the starting conformation, loosing the tightly bound water molecule in the loop-2. These data show that the structural changes elicited by CT1 molecule upon incorporation into DPC micelle take place likely in the lipid membrane, although the mode of the interaction of this toxin with DPC micelle (with the tips of the all three loops) is different from its mode in POPC membrane (primarily with the tip of the loop-1 and both the tips of the loop-1 and loop-2).

Spatial structure of the transmembrane domain heterodimer of ErbB1 and ErbB2 receptor tyrosine kinases.

Growth factor receptor tyrosine kinases of the ErbB family play a significant role in vital cellular processes and various cancers. During signal transduction across plasma membrane, ErbB receptors are involved in lateral homodimerization and heterodimerization with proper assembly of their extracellular single-span transmembrane (TM) and cytoplasmic domains. The ErbB1/ErbB2 heterodimer appears to be the strongest and most potent inducer of cellular transformation and mitogenic signaling compared to other ErbB homodimers and heterodimers. Spatial structure of the heterodimeric complex formed by TM domains of ErbB1 and ErbB2 receptors embedded into lipid bicelles was obtained by solution NMR. The ErbB1 and ErbB2 TM domains associate in a right-handed alpha-helical bundle through their N-terminal double GG4-like motif T(648)G(649)X(2)G(652)A(653) and glycine zipper motif T(652)X(3)S(656)X(3)G(660), respectively. The described heterodimer conformation is believed to support the juxtamembrane and kinase domain configuration corresponding to the receptor active state. The capability for multiple polar interactions, along with hydrogen bonding between TM segments, correlates with the observed highest affinity of the ErbB1/ErbB2 heterodimer, implying an important contribution of the TM helix-helix interaction to signal transduction.

Unique dimeric structure of BNip3 transmembrane domain suggests membrane permeabilization as a cell death trigger.

BNip3 is a prominent representative of apoptotic Bcl-2 proteins with rather unique properties initiating an atypical programmed cell death pathway resembling both necrosis and apoptosis. Many Bcl-2 family proteins modulate the permeability state of the outer mitochondrial membrane by forming homo- and hetero-oligomers. The structure and dynamics of the homodimeric transmembrane domain of BNip3 were investigated with the aid of solution NMR in lipid bicelles and molecular dynamics energy relaxation in an explicit lipid bilayer. The right-handed parallel helix-helix structure of the domain with a hydrogen bond-rich His-Ser node in the middle of the membrane, accessibility of the node for water, and continuous hydrophilic track across the membrane suggest that the domain can provide an ion-conducting pathway through the membrane. Incorporation of the BNip3 transmembrane domain into an artificial lipid bilayer resulted in pH-dependent conductivity increase. A possible biological implication of the findings in relation to triggering necrosis-like cell death by BNip3 is discussed.