These observations suggest that, unlike for their cognate binding partners, binding of C1q/MBL to BMP-1/mTLL-1 either requires a conformational change induced by immobilization or is a cooperative phenomenon involving multiple BMP-1/mTLL-1 monomers immobilized in close proximity on the sensor chip.

 BMP-1 interacted with C1q and MBL with high affinity, as reflected by affinity constants (K D) in the nM range (0.48 and 1.3 nM, respectively), the C1q/BMP-1 complex being slightly more stable than the MBL/BMP-1 complex, as indicated by a 4-fold lower dissociation rate constant (Table1).

 Kinetic analyses of the interaction of C1q or MBL with immobilized BMP-1 or mTLL-1 and competition with soluble mTLL-1 for C1q or MBL binding to immobilized mTLL-1.

 These observations suggest that BMP-1/mTLL-1 binding to C1q and MBL involve similar interactions to those between these defence collagens and their cognate binding partners, mostly likely involving CUB domains.

 Kinetic and equilibrium dissociation constants for binding of C1q and MBL to immobilized BMP-1 and mTLL-1.

 Here we describe interactions between C1q and MBL and the BTPs BMP-1 and mTLL-1 that provide further support for such a functional connection.

 To test the possibility of an interaction of C1q with BMP-1 in physiological conditions, we investigated the expression of these two proteins in human skin sections by immunofluorescence.

 Both C1q and MBL bound dose-dependently to immobilized BMP-1 (Fig.2A,B) and mTLL-1 (Fig.2C,D).

 To investigate whether BMP-1 and mTLL-1 bind to homologous regions within C1q and MBL, we investigated the capacity of the C1s-C1r-C1r-C1s tetramer and the MASP-2 dimer to compete for binding of C1q and MBL to the BTPs.

 Competition by soluble proteases for binding of C1q and MBL to immobilized BMP-1 or mTLL-1.