Nature 385, 650 - 653 (13 February 1997); doi:10.1038/385650a0

Activation of the Sire-family tyrosine kinase Hck by SH3 domain displacement

Ismail Moarefi^*†§, Michelle LaFevre-Bernt^‡§, Frank Sicheri^*, Morgan Huse^*, Chi-Hon Lee^*, John Kuriyan^*† & W. Todd Miller^‡

^* Laboratories of Molecular Biophysics,^† Howard Hughes Medical Institute, The Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
^‡ Department of Physiology and Biophysics, School of Medicine, State University of New York at Stony Brook, Stony Brook, New York 11794, USA
^§ These authors contributed equally to this work.

The protein Hck is a member of the Src family of non-receptor tyrosine kinases which is preferentially expressed in haematopoietic cells of the myeloid and B-lymphoid lineages^1,2. Src kinases are inhibited by tyrosine-phosphorylation at a carboxy-terminal site^3–9. The SH2 domains of these enzymes play an essential role in this regulation by binding to the tyrosine-phosphorylated tail^8–11. The crystal structure of the downregulated form of Hck has been determined¹² and reveals that the SH2 domain regulates enzymatic activity indirectly; intramolecular interactions between the SH3 and catalytic domains appear to stabilize an inactive form of the kinase. Here we compare the roles of the SH2 and SH3 domains in modulating the activity of Hck in an investigation of the C-terminally phosphorylated form of the enzyme. We show that addition of the HIV-1 Nef protein, which is a high-affinity ligand for the Hck SH3 domain, to either the downregulated or activated form of Hck causes a large increase in Hck catalytic activity. The intact SH3-binding motif in Nef is crucial for Hck activation. Our results indicate that binding of the Hck SH3 domain by Nef causes a more marked activation of the enzyme than does binding of the SH2 domain, suggesting a new mechanism for regulation of the activity of tyrosine kinases.

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