PIP2 promotes conformation-specific dimerization of the EphA2 membrane region

Katherine M Stefanski,Charles M Russell,Justin M Westerfield,Rajan Lamichhane,Francisco N Barrera

doi:10.1074/jbc.ra120.016423

Katherine M Stefanski, Charles M Russell + Show 3 more

Open Access

https://doi.org/10.1074/jbc.ra120.016423

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Abstract

The impact of the EphA2 receptor on cancer malignancy hinges on the two different ways it can be activated. EphA2 induces antioncogenic signaling after ligand binding, but ligand-independent activation of EphA2 is pro-oncogenic. It is believed that the transmembrane (TM) domain of EphA2 adopts two alternate conformations in the ligand-dependent and the ligand-independent states. However, it is poorly understood how the difference in TM helical crossing angles found in the two conformations impacts the activity and regulation of EphA2. We devised a method that uses hydrophobic matching to stabilize two conformations of a peptide comprising the EphA2 TM domain and a portion of the intracellular juxtamembrane (JM) segment. The two conformations exhibit different TM crossing angles, resembling the ligand-dependent and ligand-independent states. We developed a single-molecule technique using styrene maleic acid lipid particles to measure dimerization in membranes. We observed that the signaling lipid PIP2 promotes TM dimerization, but only in the small crossing angle state, which we propose corresponds to the ligand-independent conformation. In this state the two TMs are almost parallel, and the positively charged JM segments are expected to be close to each other, causing electrostatic repulsion. The mechanism PIP2 uses to promote dimerization might involve alleviating this repulsion due to its high density of negative charges. Our data reveal a conformational coupling between the TM and JM regions and suggest that PIP2 might directly exert a regulatory effect on EphA2 activation in cells that is specific to the ligand-independent conformation of the receptor.

Highlights

Implicated in skeletal malformations that cause cleft palate [7]
These findings, combined with the structural data, gave rise to the following model: In the ligandindependent active state, the EphA2 transmembrane domain (TMD) dimerizes via the heptad repeat (HR) motif with nearly parallel TMDs, while in the liganddependent signaling state, EphA2 dimerizes via the glycine zipper (GZ) with tilted TMDs [28, 29]
We used the TMJM peptide, which comprises a short stretch of extracellular residues, the TMD, and first five JM residues of EphA2 (Fig. 1A)

Summary

Introduction

Implicated in skeletal malformations that cause cleft palate [7]. a large body of research exists establishing that Eph receptors are overexpressed in a variety of cancer types. We examine how bilayer composition and the position of JM residues affect self-assembly of the TMD of EphA2, using a novel single-molecule fluorescence approach in styrene maleic acid lipid particles (SMALPs). These data, combined with the tryptophan fluorescence results (Fig. 3A), led us to conclude that helical tilt alone does not significantly affect the association of the JM residues of TMJM with PC lipid bilayers.

Results

Conclusion