T-cell receptor (TCR) signaling is initiated by recruiting ZAP-70 to the cytosolic part of TCR. ZAP-70, a non-receptor tyrosine kinase, is composed of an N-terminal tandem SH2 (tSH2) domain connected to the C-terminal kinase domain. The ZAP-70 is recruited to the membrane through binding of tSH2 domain and the doubly phosphorylated ITAM motifs of CD3 chains in the TCR complex. Our results show that the tSH2 domain undergoes a biphasic structural transition while binding to the doubly phosphorylated ITAM-ζ1 peptide. The C-terminal SH2 domain binds first to the phosphotyrosine residue of ITAM peptide to form an encounter complex leading to subsequent binding of second phosphotyrosine residue to the N-SH2 domain. We decipher a network of noncovalent interactions that allosterically couple the two SH2 domains during binding to doubly phosphorylated ITAMs. Mutation in the allosteric network residues, for example, W165C, uncouples the formation of encounter complex to the subsequent ITAM binding thus explaining the altered recruitment of ZAP-70 to the plasma membrane causing autoimmune arthritis in mice. The proposed mechanism of allosteric coupling is unique to ZAP-70, which is fundamentally different from Syk, a close homolog of ZAP-70 expressed in B-cells.
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Nanobodies reset the time of the bacterial LRRK2 cycle. A bacterial homologue of the Parkinson's disease (PD)-associated protein LRRK2 cycles between a dimeric and monomeric state concomitant with GTP binding and hydrolysis, and certain PD mutations disrupt this cycle by stabilizing the dimer. In this issue Leemans and co-workers (pp. 1203–1218) report the identification and characterization of a Nanobody that can allosterically modulate this GTPase cycle, thereby "resetting" the deregulating effect of PD mutations. The front image shows the structure of the bacterial LRRK2 that we previously solved in the background, with in the foreground the bacterial LRRK2 dimer/monomer cycle represented as a clock that is being reset by the Nanobody shown as the arrows of the clock hands. The image was created by Christian Galicia and provided by Wim Versées.
An allosteric hot spot in the tandem-SH2 domain of ZAP-70 regulates T-cell signaling
Kaustav Gangopadhyay, Bharat Manna, Swarnendu Roy, Sunitha Kumari, Olivia Debnath, Subhankar Chowdhury, Amit Ghosh, Rahul Das; An allosteric hot spot in the tandem-SH2 domain of ZAP-70 regulates T-cell signaling. Biochem J 17 April 2020; 477 (7): 1287–1308. doi: https://doi.org/10.1042/BCJ20190879
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