ROR and RYK extracellular region structures suggest that receptor tyrosine kinases have distinct WNT-recognition modes.

WNTs play key roles in development and disease, signaling through Frizzled (FZD) seven-pass transmembrane receptors and numerous co-receptors including ROR and RYK family receptor tyrosine kinases (RTKs). We describe crystal structures and WNT-binding characteristics of extracellular regions from the Drosophila ROR and RYK orthologs Nrk (neurospecific receptor tyrosine kinase) and Derailed-2 (Drl-2), which bind WNTs though a FZD-related cysteine-rich domain (CRD) and WNT-inhibitory factor (WIF) domain respectively. Our crystal structures suggest that neither Nrk nor Drl-2 can accommodate the acyl chain typically attached to WNTs. The Nrk CRD contains a deeply buried bound fatty acid, unlikely to be exchangeable. The Drl-2 WIF domain lacks the lipid-binding site seen in WIF-1. We also find that recombinant DWnt-5 can bind Drosophila ROR and RYK orthologs despite lacking an acyl chain. Alongside analyses of WNT/receptor interaction sites, our structures provide further insight into how WNTs may recruit RTK co-receptors into signaling complexes.

Comparison of Saccharomyces cerevisiae F-BAR domain structures reveals a conserved inositol phosphate binding site.

F-BAR domains control membrane interactions in endocytosis, cytokinesis, and cell signaling. Although they are generally thought to bind curved membranes containing negatively charged phospholipids, numerous functional studies argue that differences in lipid-binding selectivities of F-BAR domains are functionally important. Here, we compare membrane-binding properties of the Saccharomyces cerevisiae F-BAR domains in vitro and in vivo. Whereas some F-BAR domains (such as Bzz1p and Hof1p F-BARs) bind equally well to all phospholipids, the F-BAR domain from the RhoGAP Rgd1p preferentially binds phosphoinositides. We determined X-ray crystal structures of F-BAR domains from Hof1p and Rgd1p, the latter bound to an inositol phosphate. The structures explain phospholipid-binding selectivity differences and reveal an F-BAR phosphoinositide binding site that is fully conserved in a mammalian RhoGAP called Gmip and is partly retained in certain other F-BAR domains. Our findings reveal previously unappreciated determinants of F-BAR domain lipid-binding specificity and provide a basis for its prediction from sequence.

Receptor tyrosine kinases with intracellular pseudokinase domains.

As with other groups of protein kinases, approximately 10% of the RTKs (receptor tyrosine kinases) in the human proteome contain intracellular pseudokinases that lack one or more conserved catalytically important residues. These include ErbB3, a member of the EGFR (epidermal growth factor receptor) family, and a series of unconventional Wnt receptors. We showed previously that, despite its reputation as a pseudokinase, the ErbB3 TKD (tyrosine kinase domain) does retain significant, albeit weak, kinase activity. This led us to suggest that a subgroup of RTKs may be able to signal even with very inefficient kinases. Recent work suggests that this is not the case, however. Other pseudokinase RTKs have not revealed significant kinase activity, and mutations that impair ErbB3's weak kinase activity have not so far been found to exhibit signalling defects. These findings therefore point to models in which the TKDs of pseudokinase RTKs participate in receptor signalling by allosterically regulating associated kinases (such as ErbB3 regulation of ErbB2) and/or function as regulated 'scaffolds' for other intermolecular interactions central to signal propagation. Further structural and functional studies, particularly of the pseudokinase RTKs involved in Wnt signalling, are required to shed new light on these intriguing signalling mechanisms.

Assessing the range of kinase autoinhibition mechanisms in the insulin receptor family.

To investigate the range of autoinhibitory mechanisms used by TKDs (tyrosine kinase domains) from the insulin receptor family of RTKs (receptor tyrosine kinases), we determined crystal structures of TKDs from TrkA (tropomyosin receptor kinase A, a nerve growth factor receptor) and Ror2 (receptor tyrosine kinase-like orphan receptor 2, an unconventional Wnt receptor). TrkA autoinhibition closely resembles that seen for the insulin receptor, relying on projection of an activation loop tyrosine residue into the substrate-binding site and occlusion of the ATP-binding site by the activation loop. Ror2 employs similar mechanisms, but the unusual replacement of the phenylalanine residue in its Asp-Phe-Gly motif with leucine necessitates occlusion of the ATP-binding site by other means. The unusual Asp-Leu-Gly motif in Ror2 is displaced compared with other inactive kinases, allowing the activation loop to interact directly with the TKD's αC helix, in another mode of autoinhibition that is characteristic of the other extreme of this receptor family: ALK (anaplastic lymphoma kinase) and Met. These findings provide insight into the expected range of activating mutations in these TKDs in cancer. We also describe symmetrical dimers of the inactive TrkA TKD resembling those found in other RTKs, possibly reflecting an arrangement of kinase domains in a pre-formed TrkA dimer.

Kinase associated-1 domains drive MARK/PAR1 kinases to membrane targets by binding acidic phospholipids.

Phospholipid-binding modules such as PH, C1, and C2 domains play crucial roles in location-dependent regulation of many protein kinases. Here, we identify the KA1 domain (kinase associated-1 domain), found at the C terminus of yeast septin-associated kinases (Kcc4p, Gin4p, and Hsl1p) and human MARK/PAR1 kinases, as a membrane association domain that binds acidic phospholipids. Membrane localization of isolated KA1 domains depends on phosphatidylserine. Using X-ray crystallography, we identified a structurally conserved binding site for anionic phospholipids in KA1 domains from Kcc4p and MARK1. Mutating this site impairs membrane association of both KA1 domains and intact proteins and reveals the importance of phosphatidylserine for bud neck localization of yeast Kcc4p. Our data suggest that KA1 domains contribute to "coincidence detection," allowing kinases to bind other regulators (such as septins) only at the membrane surface. These findings have important implications for understanding MARK/PAR1 kinases, which are implicated in Alzheimer's disease, cancer, and autism.

ErbB3/HER3 does not homodimerize upon neuregulin binding at the cell surface.

To understand signaling by the neuregulin (NRG) receptor ErbB3/HER3, it is important to know whether ErbB3 forms homodimers upon ligand binding. Previous biophysical studies suggest that the ErbB3 extracellular region remains monomeric when bound to NRG. We used a chimeric receptor approach to address this question in living cells, fusing the extracellular region of ErbB3 to the kinase-active intracellular domain of ErbB1. The ErbB3/ErbB1 chimera responded to NRG only if ErbB2 was co-expressed in the same cells, whereas an ErbB4/ErbB1 chimera responded without ErbB2. We, therefore, suggest that ErbB3 is an obligate heterodimerization partner because of its inability to homodimerize.

Genome-wide analysis of membrane targeting by S. cerevisiae pleckstrin homology domains.

Pleckstrin homology (PH) domains are small protein modules known for their ability to bind phosphoinositides and to drive membrane recruitment of their host proteins. We investigated phosphoinositide binding (in vitro and in vivo) and subcellular localization, and we modeled the electrostatic properties for all 33 PH domains encoded in the S. cerevisiae genome. Only one PH domain (from Num1p) binds phosphoinositides with high affinity and specificity. Six bind phosphoinositides with moderate affinity and little specificity and are membrane targeted in a phosphoinositide-dependent manner. Although all of the remaining 26 yeast PH domains bind phosphoinositides very weakly or not at all, three were nonetheless efficiently membrane targeted. Our proteome-wide analysis argues that membrane targeting is important for only approximately 30% of yeast PH domains and is defined by binding to both phosphoinositides and other targets. These findings have significant implications for understanding the function of proteins that contain this common domain.

EGF activates its receptor by removing interactions that autoinhibit ectodomain dimerization.

Epidermal growth factor (EGF) receptor is the prototype of the ErbB (HER) family receptor tyrosine kinases (RTKs), which regulate cell growth and differentiation and are implicated in many human cancers. EGF activates its receptor by inducing dimerization of the 621 amino acid EGF receptor extracellular region. We describe the 2.8 A resolution crystal structure of this entire extracellular region (sEGFR) in an unactivated state. The structure reveals an autoinhibited configuration, where the dimerization interface recently identified in activated sEGFR structures is completely occluded by intramolecular interactions. To activate the receptor, EGF binding must promote a large domain rearrangement that exposes this dimerization interface. This contrasts starkly with other RTK activation mechanisms and suggests new approaches for designing ErbB receptor antagonists.

The single transmembrane domains of ErbB receptors self-associate in cell membranes.

Members of the epidermal growth factor receptor, or ErbB, family of receptor tyrosine kinases have a single transmembrane (TM) alpha-helix that is usually assumed to play a passive role in ligand-induced dimerization and activation of the receptor. However, recent studies with the epidermal growth factor receptor (ErbB1) and the erythropoietin receptor have indicated that interactions between TM alpha-helices do contribute to stabilization of ligand-independent and/or ligand-induced receptor dimers. In addition, not all of the expected ErbB receptor ligand-induced dimerization events can be recapitulated using isolated extracellular domains, suggesting that other regions of the receptor, such as the TM domain, may contribute to dimerization in vivo. Using an approach for analyzing TM domain interactions in Escherichia coli cell membranes, named TOXCAT, we find that the TM domains of ErbB receptors self-associate strongly in the absence of their extracellular domains, with the rank order ErbB4-TM > ErbB1-TM equivalent to ErbB2-TM > ErbB3-TM. A limited mutational analysis suggests that dimerization of these TM domains involves one or more GXXXG motifs, which occur frequently in the TM domains of receptor tyrosine kinases and are critical for stabilizing the glycophorin A TM domain dimer. We also analyzed the effect of the valine to glutamic acid mutation in ErbB2 that constitutively activates this receptor. Contrary to our expectations, this mutation reduced rather than increased ErbB2-TM dimerization. Our findings suggest a role for TM domain interactions in ErbB receptor function, possibly in stabilizing inactive ligand-independent receptor dimers that have been observed by several groups.