Interactions between mannose-binding lectin and MASPs during complement activation by the lectin pathway

Abstract

The lectin pathway of complement performs a key role within the immune system by recognising pathogens through patterns of sugar moieties displayed on their cell surfaces and neutralising them via an antibody-independent reaction cascade. While particularly important during early childhood before the adaptive immune system is established, or when adaptive immunity is compromised, it has a protective function throughout life, neutralising invading pathogens directly and helping to stimulate and direct an effective immune response. Complement activation is initiated when complexes comprising mannose-binding lectin (MBL) or serum ficolins and MBL-associated serine protease-2 (MASP-2) bind to pathogens. Binding induces conformational changes in these complexes, leading to autoactivation of the MASPs, which in turn activate the downstream reaction cascade. A major goal in complement research is to understand the molecular events that trigger complement activation. Over the last few years, structure-function studies have improved our knowledge of the way in which MBL binds to MASPs by defining the portions of these proteins that interact and by solving the structures of key protein fragments. In this review, I will summarise the main findings of these studies and describe current theories to explain how the components combine to initiate the reaction cascade.

Fig. 1

Domain organisation of MBL and MASPs. (A) Structural organisation of MBL. The collagenous domain is shown as two segments, which are separated by the interruption in the Gly–Xaa–Yaa collagen repeat. Arrows show potentially flexible regions. (B) Structural organisation of MASPs. Regions that mediate interactions with MBL are shown in light grey. Regions involved in substrate recognition are shown in dark grey. The position of the cleavage site for zymogen activation is marked by an arrow. The solid line indicates the disulphide bond that links the N- and C-terminal fragments together following activation. Arrows above the figure show potentially flexible regions.

Fig. 2

Structures of human MAp19 (top) and the CUB1–EGF–CUB2 fragment of rat MASP-2 (bottom). The probable location of the high-affinity binding sites for MBL subunits is shown by circles, the diameters of which roughly correspond to that of a collagen triple helix. Additional low-affinity sites are probably located on each of the CUB2 domains.

Fig. 3

Aligned sequences of the CUB and EGF domains of MASP-2 and MASP-1/3. Residues providing coordination ligands for Ca²⁺ in the EGF (site I) and CUB1 domains (site II) of human Map19 are marked by (◊) and (▴), respectively. In the rat CUB1–EGF–CUB2 structure the only coordination ligands observed in the structure are provided by main chain carbonyl oxygen atoms of Val120 and Tyr140 and the side chains of Asp119 and Asn139. Residues proposed to form part of the binding site for MBL in the CUB1 domain of human Map19 are indicated by (#). The position of the asparagine residue that is hydroxylated on the β carbon within the EGF-like domain of rat MBL is marked by (○). Residues at key positions that are identical to those in human MASP-2 are shaded.

Fig. 4

Proposed location of MASP-binding sites within the collagenous domain of MBL and ficolins. Top, domain organisation of MBLs and ficolins highlighting the positions of the MASP binding sites and potential glycosylation sites. Bottom, aligned sequences of human and rat MBLs and ficolins. The consensus motif for MASP binding present in MBLs and ficolins is shaded. Positions of hydroxyproline (O) and glycosylated hydroxylysine (K) residues are based on the sequences of rat MBLs, in which all proline and lysine residues at the third position of the Gly–Xaa–Yaa are at least partially modified, with the exception of the proline residue in repeat 5, which was completely unmodified by Edman degradation (Wallis and Drickamer, 1997; Wallis and Drickamer, 1999).

Fig. 5

Model of MASP activation by MBL. Left, MASP dimer bridges two MBL subunits (the outermost subunits in this example) through equivalent interactions between each protomer of the MASP and the collagen-like domain of separate MBL subunits. Middle, binding to a bacterial surface induces a change in the angle between MBL subunits, leading to a change in the conformation of the MASP, increasing the likelihood of reciprocal cleavage of the linker region of one MASP protomer by the protease domain of the other. Right, the protease is then locked into the activated conformation and is able to cleave and activate its downstream substrates.