title: The multiligand-binding protein gC1qR, putative C1q receptor, is a mitochondrial protein.
abstract: A protein of 33 kDa (p33) that tightly binds to the globular domains of the first complement component, C1q, is thought to serve as the major C1q receptor (gC1qR) on B cells, neutrophils, and mast cells. However, the cellular routing and the subcellular localization of p33/gC1qR are unknown. We have performed confocal laser-scanning microscopy and found that p33/gC1qR is present in intracellular compartments, where it colocalizes with the mitochondrial marker protein, pyruvate dehydrogenase. No surface staining for p33/gC1qR on endothelial EA.hy926 cells was observed. A fusion protein of the p33/gC1qR presequence with green fluorescent protein translocated to the mitochondria of transfected COS-7 cells. Concomitantly, a 6-kDa portion of the fusion protein was proteolytically removed. The 33 amino-terminal residues of the presequence proved sufficient to direct reporter constructs to mitochondria. Association of p33/gC1qR with mitoplasts indicated that the mature protein of 209 residues resides in the matrix and/or the inner membrane of mitochondria. Immunocytochemistry of fetal mice tissues revealed a ubiquitous expression of p33/gC1qR, most prominently in tissues that are rich in mitochondria. Thus, the candidate complement receptor p33/gC1qR of intact cells cannot interact with plasma C1q due to mutually exclusive localizations of the components. The functional role of p33/gC1qR needs to be reconsidered.
authors.affiliation: Institute for Physiological Chemistry and Pathobiochemistry, Johannes Gutenberg University at Mainz, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Dedio
authors.forename: W
authors.initials: W
authors.lastname: Jahnen-Dechent
authors.forename: M
authors.initials: M
authors.lastname: Bachmann
authors.forename: W
authors.initials: W
authors.lastname: Müller-Esterl
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000276:immunology; D002352:Carrier Proteins; D002451:Cell Compartmentation / Q000276:immunology; D002460:Cell Line; D015922:Complement C1q / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D005333:Fetus; D049452:Green Fluorescent Proteins; D006801:Humans; D018960:Hyaluronan Receptors; D008024:Ligands; D008164:Luminescent Proteins / Q000378:metabolism; D008562:Membrane Glycoproteins; D051379:Mice; D008928:Mitochondria / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D024101:Mitochondrial Proteins; D008970:Molecular Weight; D011485:Protein Binding / Q000276:immunology; D014176:Protein Biosynthesis / Q000276:immunology; D011498:Protein Precursors / Q000096:biosynthesis / Q000235:genetics; D011499:Protein Processing, Post-Translational / Q000276:immunology; D011768:Pyruvate Dehydrogenase Complex / Q000378:metabolism; D011951:Receptors, Complement / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D018411:Spodoptera
chemical_list: C087299:C1QBP protein, human; C490193:C1qbp protein, mouse; D002352:Carrier Proteins; D018076:DNA, Complementary; D018960:Hyaluronan Receptors; D008024:Ligands; D008164:Luminescent Proteins; D008562:Membrane Glycoproteins; D024101:Mitochondrial Proteins; D011498:Protein Precursors; D011768:Pyruvate Dehydrogenase Complex; D011951:Receptors, Complement; C032402:complement 1q receptor; D049452:Green Fluorescent Proteins; D015922:Complement C1q