Eph Receptors and Ephrins Are Key Regulators of Morphogenesis 139
to the intracellular domain of the TrkB receptor tyrosine kinase. These
results show conclusively that ligand-binding specificity resides in the
N-terminal globular domain. Recently, the crystal structure of the N-terminal globular domain of EphB2 was solved (Himanen et al. 1998). The
domain folds into a compact jellyroll p-sandwich composed of two
antiparallel p-sheets and has structural similarities with the carbohydrate binding domain of lectins and influenza virus hemagglutinin.
Structure-based mutagenesis identified an extended loop packed against
the concave p-sandwich surface as important for ligand-binding and
subclass specificity.
Adjacent to the N-terminal domain is an EGF-like, cysteine-rich
region of unknown function and two fibronectin type III repeats. Such
fibronectin type III repeats appear in ectodomains of numerous cell
adhesion molecules, receptor tyrosine kinases and receptor tyrosine
phosphatases and may be involved in dimerisation. In fact, incubation of
cells with divalent complexes of the EGF-like region plus the two
fibronectin type III repeats of EphA3 caused ligand-independent EphA3
receptor transphosphorylation suggesting the presence of a dimerisation
motif (Lackmann et al. 1998). It was suggested that Eph receptor activation occurs by a two-step mechanism, with distinct ligand binding via
the N-terminal globular domain followed by receptor-receptor oligomerisation via the more C-terminal dimerisation domain (Lackmann
et al. 1998).
The C-terminal intracellular region of the protein includes the kinase
domain. A highly conserved motif containing two tyrosine residues is
found in the juxtamembrane intracellular region of all Eph receptors
(Ellis et al. 1996; Holland et al. 1997). These tyrosine residues are also
major in vitro autophosphorylation sites for EphA4 (Ellis et al. 1996)
and EphB2 (Holland et al. 1997), and are likely to be important for
intracellular signalling. It has been shown that a number of SH2 domain
cytoplasmic proteins bind to the juxtamembrane region of the receptor
when it is activated. These include the Src-like tyrosine kinases p59fyn
and p60src which bind to this region in EphA4 (Ellis et al. 1996) and
EphB2, respectively (Zisch et al. 1998). The Ras GTPase activating
protein (RasGAP) binds through its SH2 domain to tyrosine phosphorylated EphB2, as does a 62-64 kDa protein p62dok and the SH2/SH3
domain adaptor protein Nck. It is likely that the RasGAP, p62dok and
Nck proteins form a complex bound to the juxtamembrane region of
to the intracellular domain of the TrkB receptor tyrosine kinase. These
results show conclusively that ligand-binding specificity resides in the
N-terminal globular domain. Recently, the crystal structure of the N-terminal globular domain of EphB2 was solved (Himanen et al. 1998). The
domain folds into a compact jellyroll p-sandwich composed of two
antiparallel p-sheets and has structural similarities with the carbohydrate binding domain of lectins and influenza virus hemagglutinin.
Structure-based mutagenesis identified an extended loop packed against
the concave p-sandwich surface as important for ligand-binding and
subclass specificity.
Adjacent to the N-terminal domain is an EGF-like, cysteine-rich
region of unknown function and two fibronectin type III repeats. Such
fibronectin type III repeats appear in ectodomains of numerous cell
adhesion molecules, receptor tyrosine kinases and receptor tyrosine
phosphatases and may be involved in dimerisation. In fact, incubation of
cells with divalent complexes of the EGF-like region plus the two
fibronectin type III repeats of EphA3 caused ligand-independent EphA3
receptor transphosphorylation suggesting the presence of a dimerisation
motif (Lackmann et al. 1998). It was suggested that Eph receptor activation occurs by a two-step mechanism, with distinct ligand binding via
the N-terminal globular domain followed by receptor-receptor oligomerisation via the more C-terminal dimerisation domain (Lackmann
et al. 1998).
The C-terminal intracellular region of the protein includes the kinase
domain. A highly conserved motif containing two tyrosine residues is
found in the juxtamembrane intracellular region of all Eph receptors
(Ellis et al. 1996; Holland et al. 1997). These tyrosine residues are also
major in vitro autophosphorylation sites for EphA4 (Ellis et al. 1996)
and EphB2 (Holland et al. 1997), and are likely to be important for
intracellular signalling. It has been shown that a number of SH2 domain
cytoplasmic proteins bind to the juxtamembrane region of the receptor
when it is activated. These include the Src-like tyrosine kinases p59fyn
and p60src which bind to this region in EphA4 (Ellis et al. 1996) and
EphB2, respectively (Zisch et al. 1998). The Ras GTPase activating
protein (RasGAP) binds through its SH2 domain to tyrosine phosphorylated EphB2, as does a 62-64 kDa protein p62dok and the SH2/SH3
domain adaptor protein Nck. It is likely that the RasGAP, p62dok and
Nck proteins form a complex bound to the juxtamembrane region of