Binding of the pentamer/hexamer forms of mannan-binding protein to zymosan activates the proenzyme C1r2C1s2 complex, of the classical pathway of complement, without involvement of C1q.
The serum lectin, mannan binding protein (MBP), was isolated in a yield of 40 micrograms/liter from pooled normal human serum by affinity chromatography on mannan-Sepharose, followed by gel-filtration and ion-exchange chromatography and finally by passage down an anti-IgM Sepharose column. A rabbit antiserum was prepared against the purified MBP and an enzyme-linked immunoassay developed that used both the specificity of the polyclonal antibody and the Ca+(+)-dependent carbohydrate binding property of MBP. Assay of the sera from 103 blood-donors showed a wide range of MBP levels, ranging from 0 to 870 micrograms/liter. MBP, after interaction with zymosan, caused efficient activation of a C1r2 125I-C1s2 complex that was prepared by incubation of 125I-C1s2 with serum, from a patient with a complete genetic deficiency of C1q, followed by gel-filtration on Sepharose 6B. The purified MBP is composed of a mixture of trimers, tetramers, pentamers, and hexamers of an approximate 90-kDa structural unit as judged by chromatography, SDS-PAGE and electron microscopy studies. Only the molecules in the pentamer/hexamer fraction, which have a similar overall structure to that of C1q, appeared to cause efficient, zymosan-dependent, activation of C1s within the C1r2C1s2 complex. The pentamer/hexamer form of MBP may therefore play an important role in antibody-independent activation of the C system during the early stages of certain infections.
Lu
J H
JH
Department of Biochemistry, University of Oxford, U.K.
Thiel
S
S
Wiedemann
H
H
Timpl
R
R
Reid
K B
KB
D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000097:blood / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D037222:Collectins; D003167:Complement Activation; D015922:Complement C1q / Q000378:metabolism*; D015923:Complement C1r / Q000378:metabolism*; D003173:Complement C1s / Q000378:metabolism*; D003171:Complement Pathway, Classical; D004789:Enzyme Activation; D006801:Humans; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008970:Molecular Weight; D015054:Zymosan / Q000378:metabolism*
D002352:Carrier Proteins; D037222:Collectins; D046911:Macromolecular Substances; D015922:Complement C1q; D015054:Zymosan; D015923:Complement C1r; D003173:Complement C1s; D002118:Calcium