Unravelling the mechanism of complement activation via the lectin pathway

Activation of complement is involved in the clearance of foreign pathogens, altered-self and apoptotic cells. It proceeds through the Classical, Lectin or Alternative pathways and ultimately results in destruction of the cell through lysis by the membrane attack complex. Activation of the Lectin pathway is triggered by the recognition of carbohydrate targets by families of collagenous proteins, called mannose-binding lectins (MBLs) and ficolins. These recognition proteins activate associated proteases called MBL-associated serine proteases (MASPs). Once activated, the MASPs cleave downstream targets to initiate the complement cascade. MASP-2 circulates in a complex with MBL and activates when it recognises mannose-type sugars. The exact molecular mechanisms of the activation of MASP-2 by MBL remains unclear and this is the focus of the current work. In this thesis I demonstrate that MASP binding can be introduced into pulmonary surfactant protein A (SP-A), a protein with a similar architecture to MBL, but which cannot bind MASPs to activate complement, through a series of substitutions to the collagenous domain. Surprisingly, introduction of the MASP-binding site results in constitutive activation of MASP-2, even in the absence of a carbohydrate target, thus lacking the control present in MBL. I then investigated the basis for this control by producing chimeric proteins of MBL and SP-A. The chimeras demonstrate that target-specific activation originates from a portion of MBL comprising the carbohydrate-recognition domains (CRDs) and neck region comprising an α-helical coiled coil. Additional work using a modified MBL that recognises galactose, subsequently demonstrates that the specific activity does not originate from the CRDs themselves, suggesting the neck domain plays an important role in the activation of MASPs. Further work in this thesis revolves around a newly discovered collectin, CL-K1 which is probably involved in complement activation and also important developmental processes. Mutations within the CRD of CL-K1 result in developmental disorders known as 3MC syndrome. The work in this thesis shows that one of these mutants is destabilized and defective in sugar binding hence revealing the likely molecular basis of disease. In addition, the structure of the CRD has been solved by X-ray crystallography enabling the key residues to be visualised.