PTK7 (PTK7 protein tyrosine kinase 7)

Note	The gene of PTK7 spans approximately 85 kb in human and 65 kb in mouse. These two genes shared the same structure and are composed by 20 exons (Jung et al., 2002; Jung et al., 2004).
Description	The PTK7 gene is organized in 20 exons in human and mouse (see figure below). In humans, exon 1 encodes the translation initiation codon and the signal peptide. Exons 2 to 13 encode the extracellular region. The 5'-half of exon 14 encodes the transmembrane region, and the rest of exon 14 and 5'-half of exon 15 encode the juxtamembrane region. The 3'-half of exon 15 and exons 16 to 20 encode the tyrosine kinase domain (Jung et al., 2002).
Transcription	The 883-bp 5'-flanking sequence from the ATG start codon contains a functional promoter. Several canonical binding sites for transcription factors (NFAT, SP1, dEF1, LMO2COM, v-MYB, TCF11, NF1, IK-2, AP4) are present in this sequence and might be important for expression of PTK7 (Jung et al., 2002). Moreover, a report showed that expression of PTK7 is regulated by Cdx transcription factors. Indeed, several Cdx response elements in the 5'-flanking sequence of ptk7 have been identified by chromatin immunoprecipitation analysis and activity of these sites have been demonstrated in cultured cells (Savory et al., 2011). mRNA: Alternative splicing give rise to five PTK7 isoforms in human, which have been described in testis. PTK7-1 is the full length form of PTK7 and the length of its mRNA is 3213 bp. Others isoforms have a loss of one or several exons coding for the extracellular part of the protein (i.e. immunoglobulin loops) and are summarized in the table above (Jung et al., 2002). Shorter PTK7 forms (i.e < 500 AA) have been reported in Ensembl but have not been validated by in vitro experiments.

Note	PTK7 has a classical RTK structure with an extracellular region, a single transmembrane region and an intracellular tyrosine kinase domain.
	A. Structure and organization of PTK7 gene. Each box represents an exon. B. Structure and features of PTK7 protein. Structure and mapping of each immunoglobulin-like loop domains are indicated in the figure. The main features of the protein (i.e cleavage site and non-classical residues in the catalytic domain) are mentioned in the figure. Ig: immunoglobulin loops domain.TM: transmembrane domain. JM: juxtamembrane domain.
Description	The human and mouse PTK7 proteins comprise 1070 and 1062 amino acids, respectively, and share 92.6% identity. The extracellular region is composed by seven immunoglobulin-like domains. Although the kinase domain is catalytically inactive, it is highly evolutionarily conserved during evolution. Three major non-classical sequences have been described: the GXGXXG ATP binding motif is changed in GXSXXG. The HDRL motif required for the catalytic proton transfer is changed in a HKDL motif, and the aspartate residue of the DFG motif usually coordinating Mg2+-ATP binding is changed to an uncharged alanine residue (ALG motif) (Jung et al., 2004; Boudeau et al., 2006). No convincing experimental data support yet a catalytic function of the PTK7 tyrosine kinase domain.
Expression	PTK7 is ubiquitously expressed at low level in epithelial, endothelial and hematopoietic tissues. PTK7 is highly expressed in tumoral tissues: colon cancer (Mossie et al., 1995), melanoma (Easty et al., 1997), breast cancer (Speers et al., 2009), acute myeloid leukemia, acute lymphoid leukemia (Müller-Tidow et al., 2004; Prébet et al., 2010).
Localisation	PTK7 is localized at the plasma membrane and has its extracellular domain exposed at the cell surface. It is described that PTK7 is processed by MT1-MPP, a metalloproteinase, that releases a soluble form of PTK7. From in vivo and in vitro studies, it appears that a fine-tuned balance between full length PTK7 and soluble PTK7 is required for normal embryonic development and directional cell migration (Golubkov et al., 2010; Golubkov et al., 2011).
Function	The loss of PTK7 protein in mouse leads to a severe and lethal phenotype of neural tube defect closure (craniorachischisis) and abdomen defect closure. Also, PTK7 is implicated in planar cell polarity and in the convergent extension embryonic process (Lu et al., 2004; Yen et al., 2009). Despite the lack of tyrosine kinase activity, PTK7 is involved in epithelial and hematopoietic cell migration. PTK7 is able to interact with VEGFR1 and is implicated into angiogenesis (Shin et al., 2008; Lee et al., 2011). In term of signaling, PTK7 is able to bind to β-catenin and Dsh proteins (Shnitsar and Borchers, 2008). The formation of a tripartite complex Dsh-Rack1-Fz7 places PTK7 as an actor of non-canonical Wnt pathway (Wehner et al., 2011). Interaction with β-catenin suggests that PTK7 is also involved in canonical Wnt pathway (Puppo et al., 2011). However, the role of PTK7 as activator or inhibitor of Wnt canonical signaling is still controversial (Peradziryi et al., 2011). In Drosophila, PTK7 (also called OTK) forms a protein complex with members of the Plexin family proteins. Also, OTK is apparently also involved in the Plexin-Semaphorin pathway (Wagner et al., 2010).
Homology	The extracellular part show similarities with adhesion molecules of the immunoglobulin superfamily due to the presence of immunoglobulin loops domains. Phylogenetic study showed that among the RTKs of the immunoglobulin superfamily, ptk7 and musk genes have derived very early during evolution and have created an independent branch probably endowed with particular functions. Indeed, Musk shows the highest identity sequence with PTK7 (42%) when the extracellular domains are compared (Grassot et al., 2006).

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