Observation by electron microscopy on recombinant soluble human complement receptor type 1 (sCR1) and its derivative, aglyco-sCR1, from CHO cells

The human complement receptor type 1 (CR1, C3b/C4b receptor) has attracted keen interests as an inhibitor for inflammatory and immune system. Recently CR1 was demonstrated to suppress the hyper-acute rejection in xeno-transplantation and to cure autoimmune diseases. sCR1, a soluble form of CR1, is a recombinant protein of CR1 in which the transmembrane domain at C-terminus was cleaved off and could be over-expressed in Chinese hamster ovary (CHO) cells. Previously, we reported a novel and simple method to produce and purify sCR1 [Kato et al. Biotechnol. Bioprocess. Eng. 7 (2002) 67]. In this study, we purified the derivative of sCR1, called as aglyco-sCR1, by treating sCR1 with tunicamycin so as to remove glyco-chains or to inhibit the glycosylation on CR1 protein during cell cultivation. Both sCR1 and aglyco-sCR1 proteins were examined by transmission electron microscopy. The sCR1 molecules were monodispersed in an appropriate surfactant which showed a square shape with a side length of 11.6 nm, whereas aglyco-sCR1 showed disc-like shape with a diameter of 8.2 nm having a concave at its center and formed larger oligomeric disc-like shaped assemblies with a diameter of 19.0 nm. The lack of the glyco-chains may facilitate aglyco-sCR1 to form oligomeric disc-like architectures so as to stabilize the structure in the solution. These facts may suggest that glyco-chains play an important role in the functions of glyco-protein, and the lower activity of aglyco-sCR1 in comparison with that of sCR1 to suppress the complement activation may due to the assembly formation and the dynamic changes occurred in protein morphology.