The C1q receptors

Calreticulin is widely, abundantly and constituatively expressed; it has no transmembrane domain and it is found mainly in the endoplasmic reticulum. These features are characteristic of a house-keeping protein, not of a cell surface receptor. Moreover, the pI of the calreticulin is very low, while that of C1q is very high, suggesting that its retention on C1q affinity columns might be the result of non-specific interactions. This study sought to identify a membrane-targeted isoform of calreticulin that might represent a functional C1q receptor. Northern blot analyses, genomic Southern blots and extensive screening of cDNA libraries indicate that no such isoform exists. However, we have been able to localise the C1q binding sites within calreticulin and to demonstrate that binding is specific.;gC1qBP, a 33 kDa protein that binds to the globular heads of C1q has also been reported to be present in the plasma membrane. However, like calreticulin, gC1qBP is very acidic, raising the possibility that its binding to C1q is non-specific. Results presented here show that gC1qBP is an abundant, ubiquitous, constitutative and highly conserved protein: features of a house-keeping protein. Most importantly, our results, and those of two independent groups, indicate that gC1qBP is an intracellular protein, not a membrane protein, and therefore cannot be a functional C1q receptor. Recent work clearly shows that gC1qBP is located exclusively in the mitochondria.;C1q also binds a various components of the extracellular matrix (ECM), leading to the suggestion that it plays a role in tissue repair by immobilising cells at sites of injury or inflammation until the normal structure of the ECM is restored. We have shown that a newly identified microfibril-associated protein, MFAP4, belongs to the class of ECM proteins that bind to C1q.