The N-Terminal Region of Fibrillin-1 Mediates a Bipartite Interaction with LTBP1

Ian B Robertson,Hans F Dias,Isabelle H Osuch,Edward D Lowe,Sacha A Jensen,Christina Redfield,Penny A Handford

doi:10.1016/j.str.2017.06.003

Ian B Robertson, Hans F Dias + Show 5 more

Open Access

https://doi.org/10.1016/j.str.2017.06.003

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Journal: Structure	Publication Date: Jun 29, 2017
Citations: 16	License type: cc-by

Affiliation: University of Oxford

Abstract

SummaryFibrillin-1 (FBN1) mutations associated with Marfan syndrome lead to an increase in transforming growth factor β (TGF-β) activation in connective tissues resulting in pathogenic changes including aortic dilatation and dissection. Since FBN1 binds latent TGF-β binding proteins (LTBPs), the major reservoir of TGF-β in the extracellular matrix (ECM), we investigated the structural basis for the FBN1/LTBP1 interaction. We present the structure of a four-domain FBN1 fragment, EGF2-EGF3-Hyb1-cbEGF1 (FBN1E2cbEGF1), which reveals a near-linear domain organization. Binding studies demonstrate a bipartite interaction between a C-terminal LTBP1 fragment and FBN1E2cbEGF1, which lies adjacent to the latency-associated propeptide (LAP)/TGF-β binding site of LTBP1. Modeling of the binding interface suggests that, rather than interacting along the longitudinal axis, LTBP1 anchors itself to FBN1 using two independent epitopes. As part of this mechanism, a flexible pivot adjacent to the FBN1/LTBP1 binding site allows LTBP1 to make contacts with different ECM networks while presumably facilitating a force-induced/traction-based TGF-β activation mechanism.

Highlights

The fibrillin/latent transforming growth factor b (TGF-b) binding protein (LTBP) family of extracellular matrix (ECM) proteins are calcium-binding glycoproteins whose domain organization is dominated by multiple tandem repeats of calcium-binding epidermal growth factor-like and TGF-b binding protein-like (TB) domains (Figure 1A) (Robertson et al, 2011)
Overlapping protein fragments derived from the N terminus of FBN1 and the C terminus of LTBP1 have been bacterially expressed and refolded in vitro (Figures 1A and S1), as described previously (Robertson et al, 2013a, 2013b; Yadin et al, 2012), to probe their interaction at the molecular level and to determine a model of the interaction complex
(A) Single-cycle surface plasmon resonance (SPR) data showing the interaction of three overlapping two-domain LTBP1 constructs with three SPR flow cells coated with different FBN1 constructs

Summary

Graphical Abstract

Improving our knowledge of TGF-b regulation by matrix biomechanics is vital for understanding the biology of this enigmatic growth factor. Robertson et al present a bipartite model for the structure of the fibrillin-1-LTBP1 interaction that functions as a holdfast for TGF-b in the matrix. 2017, Structure 25, 1208–1221 August 1, 2017 a 2017 The Authors.

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