A putative molecular-activation switch in the transmembrane domain of erbB2

Abstract

Overexpression of the receptor tyrosine kinase (RTK) erbB2 (also designated neu or HER2) was implicated in causing a variety of human cancers, including mammary and ovarian carcinomas. Ligand-induced receptor dimerization is critical for stimulation of the intrinsic protein tyrosine kinase (PTK) of RTKs. It was therefore proposed that PTK activity is stimulated as a result of the reorientation of the cytoplasmic domains within receptor dimers, leading to transautophosphorylation and stimulation of enzymatic activity. Here, we propose a molecular mechanism for rotation-coupled activation of the erbB2 receptor. Using a computational exploration of conformation space of the transmembrane (TM) segments of an erbB2 homodimer, we found two stable conformations of the TM domain. We suggest that these conformations correspond to the active and inactive states of erbB2, and that the receptor molecules may switch from one conformation to the other without crossing exceedingly unfavorable states. This model provides an explanation for the biochemical and oncogenic properties of erbB2, such as the effects of erbB2 overexpression on kinase activity and cell transformation. Furthermore, the opposing effects of the neu* activating oncogenic point mutation and the Val-655-->Ile single-nucleotide polymorphism shown to be linked to reduced risk of breast cancer are explained in terms of shifts in the equilibrium between the active and inactive states of erbB2 in vivo.

Fig 1.

Multiple sequence alignment of the TM domains of the human members of the EGFR family. Highlights indicate dimerization motifs in TM domains (12, 14). Yellow corresponds to Sternberg–Gullick motifs (11) and green to motifs that are related to the GxxxG motif (12). The transforming neu* Val-664→Glu mutation in rats corresponds to a Val-659→Glu mutation in humans (shown in red). All family members except for human erbB3 contain two known dimerization motifs separated by seven positions, thus placing the two motifs on the same ridge of amino acid residues on a model α-helix (18) (Fig. 3). Position 655 in the human erbB2 (blue) exhibits a Val/Ile single-nucleotide polymorphism. The Ile variant is linked to reduced risk of contracting breast cancer (25).

Fig 2.

A score potential surface of a homodimer corresponding to erbB2's TM domain at a crossing angle of −35°. (Left) Each coordinate on the surface represents a unique conformation of the helix pair. Two minima are colored in deep blue, corresponding to two dimerization modes (14), in which either of the dimerization motifs mediates contact between the TM domains. The deeper minimum (white ellipse) corresponds to conformations where the C-terminal dimerization motif (Fig. 1) mediates contact, whereas the shallower minimum (yellow ellipse) corresponds to conformations where the N-terminal motif mediates dimerization (Fig. 3). The minima are not disconnected (red ellipse), and movement is likely between the two dimerization modes (Movie 1). Score is given in arbitrary units. (Right) Different conformations of the helix pair are tested by modulating α and β corresponding to rotations (°) of the monomers around their principal axes, and x to a sliding movement (Å) of one helix across the face of the other. In the global search method, the crossing angle Ψ (°) and z (Å), corresponding to movement across the face of the opposing helix along an axis perpendicular to x (not shown) are also modulated. In the restricted search method symmetry is enforced, so that α = β and z = 0. Also, Ψ is set to −35°, corresponding to a typical crossing angle for helices in the 4–4 class of helix packing (18).

Fig 3.

Stereoview of the ideal α-helix model of the TM domain of erbB2 used for the calculations presented in Fig. 2. Ser-656 and Gly-660 of the N-terminal dimerization motif (11) (Fig. 1) are yellow; Gly-668 and Gly-672 of the C-terminal dimerization motif (12) are green; and Val-664 is red. The monomers pack through either of the two motifs (14). The structural basis that stabilizes the two conformations is that the two motifs form relatively even surfaces on the helical face. Thus they form grooves (18) into which the other monomer may pack. Val-664 (red) is situated between the two motifs on the same ridge (18). Tight packing of this residue in the transition between the two dimerization modes (Movie 1) forms the saddle-point in Fig. 2 Left.