Recombinant Expression of the Full-length Ectodomain of LDL Receptor-related Protein 1 (LRP1) Unravels pH-dependent Conformational Changes and the Stoichiometry of Binding with Receptor-associated Protein (RAP)

LDL receptor-related protein 1 (LRP1) is a highly modular protein and the largest known mammalian endocytic receptor. LRP1 binds and internalizes many plasma components, playing multiple crucial roles as a scavenger and signaling molecule. One major challenge to studying LRP1 has been that it is difficult to express such a large, highly glycosylated, and cysteine-rich protein, limiting structural studies to LRP1 fragments. Here, we report the first recombinant expression of the complete 61 domains of the full-length LRP1 ectodomain. This advance was achieved with a multistep cloning approach and by using DNA dilutions to improve protein yields. We investigated the binding properties of LRP1 using receptor-associated protein (RAP) as a model ligand due to its tight binding interaction. The LRP1 conformation was studied in its bound and unbound state using mass spectrometry, small-angle X-ray scattering, and negative-stain electron microscopy at neutral and acidic pH. Our findings revealed a pH-dependent release of the ligand associated with a conformational change of the receptor. In summary, this investigation of the complete LRP1 ectodomain significantly advances our understanding of this important receptor and provides the basis for further elucidating the mechanism of action of LRP1 in a whole and integrated system.

Structures of Wnt-antagonist ZNRF3 and its complex with R-spondin 1 and implications for signaling

Zinc RING finger 3 (ZNRF3) and its homolog RING finger 43 (RNF43) antagonize Wnt signaling in adult stem cells by ubiquitinating Frizzled receptors (FZD), which leads to endocytosis of the Wnt receptor. Conversely, binding of ZNRF3/RNF43 to LGR4-6 – R-spondin blocks Frizzled ubiquitination and enhances Wnt signaling. Here, we present crystal structures of the ZNRF3 ectodomain and its complex with R-spondin 1 (RSPO1). ZNRF3 binds RSPO1 and LGR5-RSPO1 with micromolar affinity via RSPO1 furin-like 1 (Fu1) domain. Anonychia-related mutations in RSPO4 support the importance of the observed interface. The ZNRF3-RSPO1 structure resembles that of LGR5-RSPO1-RNF43, though Fu2 of RSPO1 is variably oriented. The ZNRF3-binding site overlaps with trans-interactions observed in 2:2 LGR5-RSPO1 complexes, thus binding of ZNRF3/RNF43 would disrupt such an arrangement. Sequence conservation suggests a single ligand-binding site on ZNRF3, consistent with the proposed competing binding role of ZNRF3/RNF43 in Wnt signaling.

Purification, crystallization and preliminary X-ray diffraction analysis of the lytic transglycosylase MltF from Escherichia coli

The lytic transglycosylase MltF from Escherichia coli is an outer-membrane-bound periplasmic protein with two domains: a C-terminal catalytic domain with a lysozyme-like fold and an N-terminal domain of unknown function that is homologous to the periplasmic substrate-binding proteins of ABC transporters. In order to investigate its structure and function, a soluble form of full-length MltF (sMltF) containing both domains and a soluble fragment containing only the N-terminal domain (sMltF-NTD) were purified and crystallized. Crystals of sMltF belonged to space group P4(3)2(1)2 or P4(1)2(1)2, with unit-cell parameters a = b = 110.8, c = 163.5 A and one or two molecules per asymmetric unit. A complete data set was collected to 3.5 A resolution. Crystals of sMltF-NTD belonged to space group P3(1)21, with unit-cell parameters a = b = 82.4, c = 75.2 A and one molecule per asymmetric unit. For sMltF-NTD, a complete native data set was collected to 2.20 A resolution. In addition, for phasing purposes, a three-wavelength MAD data set was collected to 2.5 A resolution using a bromide-soaked sMltF-NTD crystal. Using phases derived from the Br-MAD data, it was possible to build a partial model of sMltF-NTD.