Editors' ChoiceCell Biology

Rethinking “Pseudo”

Sci. Signal.  04 May 2010:
Vol. 3, Issue 120, pp. ec133
DOI: 10.1126/scisignal.3120ec133

Members of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases (RTKs) play important roles in cellular differentiation and proliferation and are often activated in human cancers. One of these family members, ErbB3, has been considered a pseudokinase based on the absence of several conserved residues required for catalytic activity in other kinases, yet biochemical evidence has been largely inconclusive. Upon ligand binding, homo- or heterodimerization of ErbB receptors allosterically activates each receptor’s intracellular kinase domain, which is followed by trans-autophosphorylation of tyrosine residues in the C-terminal regulatory domain to initiate signaling. Using recombinant protein purified from cultured insect cells, Shi et al. observed that a His-tagged version of the intracellular domain of human ErbB3 (ErbB3-ICD) underwent autophosphorylation when it was concentrated in vesicles in the presence of adenosine triphosphate (ATP) and either Mg2+ or Mn2+. The rate of incorporation of 32P from radiolabeled ATP indicated that the kinase activity of EGFR was about 1000 times greater than that of the tyrosine kinase domain of ErbB3 (ErbB3-TKD). ErbB3-TKD bound ATP with a Kd of 1.1 μM, suggesting that ErbB3 would bind ATP in vivo. The crystal structure of ATP-bound ErbB3-TKD was similar to that of inactive (non–ATP-bound) forms of EGFR and ErbB4, and mutations in key activation loop residues that activate EGFR and ErbB4 by destabilizing their inactive conformations instead inactivated ErbB3. A conserved aspartate residue that is modeled to act as the base acceptor for the phosphoryltranferase reaction in other protein tyrosine kinases is replaced by an asparagine in ErbB3, but computational modeling predicted an alternative mechanism by which ErbB3 could perform this proton transfer. This alternative mechanism was predicted to occur much more slowly than the conventional mechanism, offering a plausible explanation for the slow rate of ErbB3 catalysis. Although ErbB3 catalytic activity is weak compared with that of other RTKs, ErbB3 may remain catalytically active in situations where the activities of other ErbB family members have been attenuated by kinase inhibitors, such as lapatinib or gefitinib, which do not affect ErbB3. Given that ErbB3 has been implicated in resistance of cells to therapeutics that target its heterodimerization partner ErbB2, the kinase activity of ErbB3 may thus be a viable target for managing EGFR signaling–dependent tumors.

F. Shi, S. E. Telesco, Y. Liu, R. Radhakrishnan, M. A. Lemmon, ErbB3/HER3 intracellular domain is competent to bind ATP and catalyze autophosphorylation. Proc. Natl. Acad. Sci. U.S.A. 107, 7692–7697 (2010). [PubMed]