Factor D (FD) is an essential element of the alternative pathway of the complement
system, and it circulates predominantly in cleaved, activated form in the blood. In
resting blood, mannose-binding lectin–associated serine protease 3 (MASP-3) is the
exclusive activator of pro-FD. Similarly to FD, MASP-3 also circulates mainly in the
active form. It was not clear, however, how zymogen MASP-3 is activated. To decipher
its activation mechanism, we followed the cleavage of MASP-3 in human hirudin plasma.
Our data suggest that neither lectin pathway proteases nor any protease controlled
by C1-inhibitor are required for MASP-3 activation. However, EDTA and the general
proprotein convertase inhibitor decanoyl-RVKR-chloromethylketone completely prevented
activation of exogenous MASP-3 added to blood samples. In this study, we show that
proprotein convertase subtilisin/kexin (PCSK) 5 and PCSK6 are able to activate MASP-3
in vitro. Unlike PCSK5, PCSK6 was detected in human serum and plasma, and previously
PCSK6 had also been shown to activate corin in the circulation. In all, PCSK6 emerges
as the MASP-3 activator in human blood. These findings clarify the very first step
of the activation of the alternative pathway and also connect the complement and the
proprotein convertase systems in the blood.
