Structural Basis for the Formation of PTPRG•CNTN Complexes in Neural Tissues
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Receptor protein tyrosine phosphatase gamma (PTPRG) is a cell surface receptor expressed primarily on neurons. It combines cytoplasmic tyrosine phosphatase domains and an extracellular region that includes a carbonic anhydrase-like (CA) domain. This domain mediates binding to members of a family of neural cell adhesion molecules called contactins (CNTNs) that are expressed on neurons during development and adulthood. The ectodomains of CNTNs are organized into six N-terminal immunoglobulin domains followed by four fibronectin type III repeats (FN) and a glycophosphatidylinositol anchor. Previous work demonstrated that PTPRG interacts specifically with CNTN3-6. Here, we combine biochemical and structural approaches to further characterize the interactions between PTPRG and its cognate CNTN partners. In particular, our work indicates that PTPRG associates with CNTN3-6 with similar binding affinities. This finding is consistent with our structural analyses of PTPRG•CNTN3 and PTPRG•CNTN6 complexes suggesting that CNTN3-6 use a conserved interface to bind the CA domain of PTPRG. As a first step to determine the in vivo functions of PTPRG•CNTN complexes, we attempted to localize the sites where these receptors interact. In particular, we identified the PTPRG•CNTN3 complex in the outer segment (OS) of adult mouse retinas. Further investigation of these complexes in the OS revealed that PTPRG and CNTN3 form complexes when expressed on the same cell (cis interactions). However, we also performed cell-aggregation assays indicating that PTPRG and CNTN3 can associate when expressed on distinct cells (trans interactions). To explain how the PTPRG•CNTN3 complex could form in these two distinct geometries, we analyzed the conformations taken by the CNTN3 ectodomain. In particular, our work indicates that the FN1-FN3 of CNTN3 adopts a bent conformation suggesting that the CNTN3 ectodomain bends sharply between FN2 and FN3 domains and then extends in parallel to the cell surface. Importantly, this bent conformation is found in all six CNTN family members suggesting that all CNTNs might lie parallel to the cell membrane. This orientation of CNTN ectodomains would accommodate the formation of cis and trans PTPRG•CNTN complexes. Finally, we discuss the implications of our results in PTPG/CNTN-mediated signaling.
Table of Contents
Introduction -- Materials and methods -- Structural basis for the conserved interaction of CNTN3, 4, 5 and 6 with the CA domain of PTPRG -- Structural analyses of CNTN ectodomains reveal an unexpected bent conformation -- Final discussion -- Appendix