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T. Váradi, M. Schneider, E. Sevcsik, D. Kiesenhofer, F. Baumgart, G. Batta, T. Kovács, R. Platzer, J. Huppa, J. Szöllösi, G. Schütz, M. Brameshuber, P. Nagy:
"Homo- and Heteroassociations Drive Activation of ErbB3";
Biophysical Journal, 117 (2019), 1935 - 1947.

Dimerization or the formation of higher-order oligomers is required for the activation of ErbB receptor tyrosine kinases.
The heregulin (HRG) receptor,ErbB3,must heterodimerize with other members of the family, preferentially ErbB2, to forma functional
signal transducing complex.Here,weapplied singlemolecule imaging capable of detecting long-lived and mobile associations tomeasure
their stoichiometry and mobility and analyzed data from experiments globally, taking the different lateral mobility of monomeric
and dimeric molecular species into account. Although ErbB3 was largely monomeric in the absence of stimulation and ErbB2 coexpression,
a small fraction was present as constitutive homodimers exhibiting a 40% lower mobility than monomers. HRG stimulation
increased the homodimeric fraction of ErbB3 significantly and reduced themobility of homodimers fourfold compared to constitutive
homodimers.Expression ofErbB2 elevated the homodimeric fraction ofErbB3 even in unstimulated cells and induced a 2-fold
reduction in the lateral mobility of ErbB3 homodimers. The mobility of ErbB2 was significantly lower than that of ErbB3, and HRG
induced a less pronounced decrease in the diffusion coefficient of all ErbB2 molecules and ErbB3/ErbB2 heterodimers than in the
mobility of ErbB3. The slower diffusion of ErbB2 compared to ErbB3 was abolished by depolymerizing actin filaments, whereas
ErbB2 expression induced a substantial rearrangement of microfilaments, implying a bidirectional interaction between ErbB2 and
actin. HRG stimulation of cells co-expressing ErbB3 and ErbB2 led to the formation of ErbB3 homodimers and ErbB3/ErbB2 heterodimers
in a competitive fashion. Although pertuzumab, an antibody binding to the dimerization armofErbB2, completely abolished the
formation of constitutive and HRG-induced ErbB3/ErbB2 heterodimers, it only slightly blocked ErbB3 homodimerization. The results
imply that a dynamic equilibrium exists between constitutive and ligand-induced homo- and heterodimers capable of shaping transmembrane
signaling.

http://dx.doi.org/10.1016/j.bpj.2019.10.001