Abstract
The zona pellucida (ZP) domain is present in extracellular proteins such as the zona pellucida proteins and tectorins and participates in the formation of polymeric protein networks. However, the ZP domain also occurs in the cytokine signaling co-receptor transforming growth factor β (TGF-β) receptor type 3 (TGFR-3, also known as betaglycan) where it contributes to cytokine ligand recognition. Currently it is unclear how the ZP domain architecture enables this dual functionality. Here, we identify a novel major TGF-β-binding site in the FG loop of the C-terminal subdomain of the murine TGFR-3 ZP domain (ZP-C) using protein crystallography, limited proteolysis experiments, surface plasmon resonance measurements and synthetic peptides. In the murine 2.7 Å crystal structure that we are presenting here, the FG-loop is disordered, however, well-ordered in a recently reported homologous rat ZP-C structure. Surprisingly, the adjacent external hydrophobic patch (EHP) segment is registered differently in the rat and murine structures suggesting that this segment only loosely associates with the remaining ZP-C fold. Such a flexible and temporarily-modulated association of the EHP segment with the ZP domain has been proposed to control the polymerization of ZP domain-containing proteins. Our findings suggest that this flexibility also extends to the ZP domain of TGFR-3 and might facilitate co-receptor ligand interaction and presentation via the adjacent FG-loop. This hints that a similar C-terminal region of the ZP domain architecture possibly regulates both the polymerization of extracellular matrix proteins and cytokine ligand recognition of TGFR-3.
Highlights
Transforming growth factor-b receptor type 3 (TGFR-3), known as betaglycan, is an ubiquitously expressed cell surface proteoglycan that serves as a co-receptor for members of the TGFb family of cystein knot growth factors, i.e. transforming growth factor b (TGF-b), activins, inhibins, growth differentiation factors (GDFs) and the bone morphogenetic proteins (BMPs)
The spectra are in agreement with a predominantly antiparallel b-sheet structure as expected from previous Circular dichroism (CD) experiments with uromodulin and as observed in the crystal structures of mouse ZP3-N, full-length chicken ZP3 and rat TGFR-3-zona pellucida (ZP)-C [11,12,13,27,28]
The identification of the binding site for TGF-b-like growth factors in TGFR-3 has been the subject of numerous mutagenesis studies [3,4,5,6,7,33,34]
Summary
Transforming growth factor-b receptor type 3 (TGFR-3), known as betaglycan, is an ubiquitously expressed cell surface proteoglycan that serves as a co-receptor for members of the TGFb family of cystein knot growth factors, i.e. TGF-bs, activins, inhibins, growth differentiation factors (GDFs) and the bone morphogenetic proteins (BMPs). Many TGF-b family members bind to TGFR-3 first, and this interaction subsequently facilitates the formation of a ternary signaling complex between the growth factor and the receptors TGFR-1 and TGFR-2. The molecular mechanism by which the co-receptor TGFR-3 initiates and facilitates the formation of the signaling competent complex is currently not well understood [1,2]. The 785 residue-long ectodomain of TGFR-3 can be subdivided into two halves of similar lengths (Fig. 1). The C-terminal half contains next to a bioinformatically delineated ZP core domain (residues 454 to 728, Fig. 1) a so-called external hydrophobic patch (EHP) that is part of a stretch of amino acids that connects the ZP domain to the membranespanning segment in TGFR-3 [8]. Crystals structures of both the full-length and the N-terminal ZP subdomain (ZP-N) have been determined for ZP3 as well as the C-terminal subdomain (ZP-C) of TGFR-3 [11,12,13]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
