Abstract
LRP1 is a large endocytic modular receptor that plays a crucial role in the scavenging of apoptotic material through binding to pattern-recognition molecules. It is a membrane anchored receptor of the LDL receptor family with 4 extracellular clusters of ligand binding modules called cysteine rich complement-type repeats that are involved in the interaction of LRP1 with its numerous ligands. Complement C1q was shown to interact with LRP1 and to be implicated in the phagocytosis of apoptotic cells. The present work aimed at exploring how these two large molecules interact at the molecular level using a dissection strategy. For that purpose, recombinant LRP1 clusters II, III and IV were produced in mammalian HEK293F cells and their binding properties were investigated. Clusters II and IV were found to interact specifically and efficiently with C1q with KDs in the nanomolar range. The use of truncated C1q fragments and recombinant mutated C1q allowed to localize more precisely the binding site for LRP1 on the collagen-like regions of C1q (CLRs), nearby the site that is implicated in the interaction with the cognate protease tetramer C1r2s2. This site could be a common anchorage for other ligands of C1q CLRs such as sulfated proteoglycans and Complement receptor type 1. The use of a cellular model, consisting in CHO LRP1-null cells transfected with full-length LRP1 or a cluster IV minireceptor (mini IV) confirmed that mini IV interacts with C1q at the cell membrane as well as full-length LRP1. Further cellular interaction studies finally highlighted that mini IV can endorse the full-length LRP1 binding efficiency for apoptotic cells and that C1q has no impact on this interaction.
Highlights
C1q is a defense collagen that is known for decades for its implication in the elimination of pathogens or altered-self bodies through the classical cascade of complement
In this study we demonstrate that the interaction of C1q with LDL receptorrelated protein 1 (LRP1) is involving clusters II and cluster IV, two regions that have been described as interacting with most of LRP1 ligands [25, 27]
We add here undoubtful confirmation, using soluble purified recombinant clusters, that the binding site for C1q is located on the complement-type repeat (CR) modular region of both clusters II and IV and that this interaction is in the same range of affinity slightly higher for cluster IV than cluster II
Summary
C1q is a defense collagen that is known for decades for its implication in the elimination of pathogens or altered-self bodies through the classical cascade of complement. C1q exposes six identical globular heads (GR) on one end, extending in six collagen stems (Collagen-Like Regions, CLR) that associate into a bundle on the other end of the molecule This particular structural arrangement is providing a wide diversity in C1q functions, with the globular heads recognizing targets that for most of them will trigger the classical complement cascade whereas the collagen regions are implicated in other non-complement functions. LRP1 is a large 600 kDa endocytic receptor that participates in several biological pathways and plays prominent role in endocytosis of a large number of unrelated ligands It is the largest member of the scavenger receptor family with an extracellular polypeptide extension composed of numerous structurally homologous modules of three types, EGF repeats, b-propeller domains and cysteine-rich calcium dependent complement type repeats called CR or LA modules (Figure 1). When processed inside the cells, LRP1 is associated with a chaperone of 39 kDa called receptor-associated protein (RAP), that binds the three
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