Proteolytic activity of Mep1p toward different complement proteins. The proteolytic activity of Mep1p was observed by incubating 3 μg of C2, C1q, C4b-binding protein (C4BP), factor H (FH), factor B (FB), IgG, properdin (FP), MBL, ficolin 1 (FCN1), ficolin 2 (FCN2), or ficolin 3 (FCN3), or 1 μg of factor D (FD) with 0.5 μg of Mep1p in 20 μl of Tris buffer for 60 min at 37 °C. The cleavage products were visualized by running the samples on 10% SDS-PAGE under reducing conditions and staining them with Coomassie Blue. The panels on the right side of each gel show % cleavage of the respective protein. Data represent mean ± S.D. of three experiments (*, p < 0.05; **, p < 0.005; ***, p < 0.0005). Mep1p showed strong proteolytic activity toward C4BP, MBL, FCN1, and FP and weak activity toward FH and IgG. Molecular weights: C2, 93,000; C1q, 25,000; C4BP (α-chain), 72,000; FH, 155,000; FB, 93,000; FCN1, 35,500; FCN2, 40,500; FCN3, 37,000; FD, 24,000; IgG H-chain, 50,000; IgG L-chain; 25,000; FP, 53,000; MBL, 33,500.