Complement C1q Interacts With LRP1 Clusters II and IV Through a Site Close but Different From the Binding Site of Its C1r and C1s-Associated Proteases

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 K_Ds 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.

Headless C1q: a new molecular tool to decipher its collagen-like functions

Complement component C1q, a soluble defense collagen, is the recognition protein of the classical complement pathway. C1q is able to recognize and interact with multiple targets and, via the subsequent activation of its cognate serine proteases C1r and C1s, initiates the complement cascade. C1q is made up of six ABC heterotrimers each containing two different functional regions, an N-terminal collagen-like region (CLR) and a C-terminal globular region (GR). These heterotrimers assemble via their N-terminal regions, resulting in the characteristic 'bouquet-like' shape of C1q with an N-terminal bundle of collagen fibers with six diverging stems each exhibiting a C-terminal globular head. The GRs are responsible for the versatile recognition of multiple C1q targets, whereas the CLRs trigger immune response through interacting with several cellular or soluble partners. We report here the generation of the first recombinant form of human C1q without its recognition globular heads. The noncollagenous domain 2 (nc2) of type IX collagen has been substituted for the C1q GR in order to control the correct registering of the collagen triple helices of C1q chains A, B, and C. The resulting CLR_nc2 recombinant protein produced in stably transfected EXPI293 mammalian cells was correctly assembled and folded, as demonstrated by mass spectrometry, mass photometry, and electron microscopy experiments. Its interaction properties were investigated using surface plasmon resonance analysis with known CLR ligands: the tetramer of C1r and C1s dimers and MBL-associated protein MAp44. Comparison with the interaction properties of native serum-derived C1q and CLR revealed that recombinant CLR_nc2 retains C1q CLR functional properties.