Expression and characterization of a 159 amino acid, N-terminal fragment of human complement component C1s

A 159 residue, N-terminal fragment of the human Cls complement component, Cls alpha(159), was expressed in the baculovirus, insect cell system. The protein was abundantly produced 3 days after infection, reaching levels as high as 40 mu g/ml in cell culture media. It had a molecular weight of 18,100 (+/- 4.9) Da by laser desorption mass spectrometry, close to the theoretical value of 18,111 Da, confirmed by sequencing. Sedimentation equilibrium and gel filtration column chromatography showed that Cls alpha(159) was a monomer in the presence of EDTA, and a dimer in the presence of Ca2+. The Cls alpha(159), dimer had a sedimentation coefficient of 3.1 S. When the Cls alpha(159)(2) was mixed with Clq, there was little or no interaction. Likewise, unactivated Clr, dimer had a sedimentation coefficient of 6.8 S, and when mixed with Clq little or no interaction was observed. When Cls alpha(159)(2) was mixed with the 6.8 S Clr(2) in Ca2+, a 7.5 S complex was formed, presumably the Cls alpha(159). Clr . Clr . Cls alpha(159) tetramer. When Clq, which migrated at 10.1 S was mixed with Cls alpha(159)(2) and Clr(2) in the presence of Ca2+, a Cl-like complex, but containing Cls alpha(159) instead of Cls, was formed which migrated at 14.0 S. This Cl-like molecule remained unactivated unless challenged with an ovalbumin-antiovalbumin immune complex. In the presence of immune complex, the Clr became activated. This suggested that the presence of the 159 amino acid Cls alpha domain, which held the Clr to the Clq, was sufficient to permit activation by an immune complex, even though the catalytic domains of Cls were not present. (C) 1998 Elsevier Science Ltd. All rights reserved.