MASP-1, a Promiscuous Complement Protease: Structure of Its Catalytic Region Reveals the Basis of Its Broad Specificity

Abstract

Mannose-binding lectin (MBL)-associated serine protease (MASP)-1 is an abundant component of the lectin pathway of complement. The related enzyme, MASP-2 is capable of activating the complement cascade alone. Though the concentration of MASP-1 far exceeds that of MASP-2, only a supporting role of MASP-1 has been identified regarding lectin pathway activation. Several non-complement substrates, like fibrinogen and factor XIII, have also been reported. MASP-1 belongs to the C1r/C1s/MASP family of modular serine proteases; however, its serine protease domain is evolutionary different. We have determined the crystal structure of the catalytic region of active MASP-1 and refined it to 2.55 Å resolution. Unusual features of the structure are an internal salt bridge (similar to one in factor D) between the S1 Asp189 and Arg224, and a very long 60-loop. The functional and evolutionary differences between MASP-1 and the other members of the C1r/C1s/MASP family are reflected in the crystal structure. Structural comparison of the protease domains revealed that the substrate binding groove of MASP-1 is wide and resembles that of trypsin rather than early complement proteases explaining its relaxed specificity. Also, MASP-1’s multifunctional behavior as both a complement and a coagulation enzyme is in accordance with our observation that antithrombin in the presence of heparin is a more potent inhibitor of MASP-1 than C1 inhibitor. Overall, MASP-1 behaves as a promiscuous protease. The structure shows that its substrate binding groove is accessible; however, its reactivity could be modulated by an unusually large 60-loop and an internal salt bridge involving the S1 Asp.

Structure-Function Analysis of Grass Clip Serine Protease Involved in Drosophila Toll Pathway Activation

“…In general, the restricted specificity of proteases (for example in the coagulation) results from a deep and narrow active site cleft. Indeed, several studies highlighted the role of the 60-loop in regulating the specificity of the proteases by shielding the active-site pocket (33)(34)(35).…”

Complement Protease MASP-1 Activates Human Endothelial Cells: PAR4 Activation Is a Link between Complement and Endothelial Function

“…MASP-1, however, resembles thrombin in its broader substrate specificity; it cleaves several protein substrates that are involved in the immune response and homeostasis. The relaxed substrate specificity of MASP-1 is reflected in its three-dimensional structure, having a substrate binding area similar to that of thrombin (33). It is well known that thrombin can activate a wide range of intracellular signaling pathways through PARs (15).…”

Complement and its role in innate and adaptive immune responses

“…MBL can bind to common carbohydrate PAMPs on Grampositive and Gram-negative bacteria and yeast, as well as on some viruses and parasites [15,16]. Similar to the C1 complex of the classical pathway, MBL is complexed with MBL-associated serine proteases (MASPs)-1, -2, and -3 which are functionally and structurally similar (although not identical, see Harmat et al [17]; Bally et al [18]; Gal et al [19]) to C1s and C1r, such that the binding of MBL to pathogenic surfaces leads to the activation of associated MASPs, cleavage of C2 and C4, and ultimately to the generation of the C3 convertase of both classical and lectin pathways, C4bC2a [12,13,[17][18][19][20][21][22][23].…”