Proper lateral dimerization of transmembrane domain of receptor tyrosine kinases is required for biochemical signal transduction across plasma membrane. Spatial structure of dimeric transmembrane domain of the growth factor receptor ErbB2 embedded into lipid bicelle was obtained by solution NMR followed by molecular dynamics relaxation in explicit lipid bilayer. ErbB2 transmembrane segments associate in a right-handed α-helical bundle through the N-terminal tandem GG4-like motif T 652 X 3 S 656

more »

... X 3 G 660 , providing explanation for pathogenic power of some oncogenic mutations. The epidermal growth factor (EGF) receptor, or ErbB, family is an important class of receptor tyrosine kinases (RTKs) involved in transmission of biochemical signals governing cell fate (1). Four human ErbB members form numerous homoand heterodimer combinations and bind different EGF-related ligands, thus performing diverse functions in a complex signaling network (2). The binding of peptide growth factors to the extracellular domain of the receptor triggers the dimerization of receptor monomers or a change in the relative orientation of monomers in preformed receptor dimers, leading to autophosphorylation of tyrosine residues in the cytoplasmic kinase domain (3, 4). Biochemical and genetic studies revealed that the single-helix transmembrane (TM) domains of ErbB play an active role in dimerization process and associate strongly in the absence of extracellular ligand-binding and cytoplasmic kinase domains (5, 6). Mutational analysis assumed that the dimerization involves consensus small-X 3 -small (so-called GG4-like) motifs, formed by residues with small side chains allowing tight helix packing (7, 8, 9) . RTK TM sequences often contain several remote GG4-like motifs, suggesting ability of their TM domains to adopt more than one conformation, e.g., upon socalled rotation-coupled activation of the receptor (4, 10, 11). Recent molecular modeling and solidstate NMR studies performed to predict the spatial structure of dimeric TM domain of human ErbB2 receptor and its rat homologue disclose two possible dimer conformations with interfaces located either at N-or C-terminus of the α-helical TM segment, employing different GG4-like motifs for dimerization (7, (11) (12) (13) . Nevertheless, an experimental spatial structure of dimeric TM domain for ErbB2 as well as for any other RTK members has not been reported so far. Here we present the high-resolution structure of homodimeric ErbB2 TM domain in membranemimicking lipid environment solved by heteronuclear NMR technique combined with molecular dynamics (MD) relaxation in explicit membrane. Our results distinguish one out of the potential conformations of the homodimer, which can be ascribed to the active state of the tyrosine kinase. Based on the analysis of the local conformation of the dimerization interface, we propose a molecular mechanism of action of some ErbB2 pro-oncogenic mutations, assuming overstabilization of the described TM domain conformation.