Clusterin (complement lysis inhibitor) forms a high density lipoprotein complex with apolipoprotein A-I in human plasma.
Clusterin/human complement lysis inhibitor (CLI) is incorporated stoichiometrically into the soluble terminal complement complex and inhibits the cytolytic reaction of purified complement components C5b-9 in vitro. Using an anti-clusterin affinity column, we found that an additional protein component with a molecular mass of 28-kDa co-purifies with clusterin from human plasma. We show by immunoblotting and amino acid sequencing that this component is apolipoprotein A-I (apoA-I). By using physiological salt buffers containing 0.5% Triton X-100, apoA-I is completely dissociated from clusterin bound to the antibody column. Free clusterin immobilized on the antibody-Sepharose selectively retains apoA-I from total human plasma. Delipidated apoA-I and to a lesser extent ultracentrifugation-purified high density lipoproteins (HDL) adsorbed to nitrocellulose also have a binding affinity for purified clusterin devoid of apoA-I. The isolated apoA-I-clusterin complex contains approximately 22% (w/w) lipids which are composed of 54% (mole/mol) total cholesterol (molar ratio of unesterified/esterified cholesterol, 0.58), 42% phospholipids, and 4% triglycerides. In agreement with the low lipid content, apoA-I-clusterin complexes are detected only in trace amounts in HDL fractions prepared by density ultracentrifugation. In free flow isotachophoresis, the purified apoA-I-clusterin complex has the same mobility as the native clusterin complex in human plasma and is found in the slow-migrating HDL fraction of fasting plasma. Our data indicate that clusterin circulates in plasma as a HDL complex, which may serve not only as an inhibitor of the lytic terminal complement cascade, but also as a regulator of lipid transport and local lipid redistribution.
Jenne
D E
DE
Institut of Biochemistry, University of Lausanne, Epalinges-sur-Lausanne, Switzerland.
Lowin
B
B
Peitsch
M C
MC
Böttcher
A
A
Schmitz
G
G
Tschopp
J
J
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000097:blood* / Q000302:isolation & purification / Q000648:ultrastructure; D002784:Cholesterol / Q000032:analysis; D002850:Chromatography, Gel; D051152:Clusterin; D004591:Electrophoresis, Polyacrylamide Gel; D006023:Glycoproteins / Q000097:blood* / Q000302:isolation & purification / Q000648:ultrastructure; D006801:Humans; D015151:Immunoblotting; D008075:Lipoproteins, HDL / Q000097:blood*; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D018832:Molecular Chaperones; D008970:Molecular Weight; D010743:Phospholipids / Q000032:analysis; D014280:Triglycerides / Q000032:analysis
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; C495242:CLU protein, human; D051152:Clusterin; D006023:Glycoproteins; D008075:Lipoproteins, HDL; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D010743:Phospholipids; D014280:Triglycerides; D002784:Cholesterol