Section 55
Chapter 54,206

MAp44, a human protein associated with pattern recognition molecules of the complement system and regulating the lectin pathway of complement activation

Degn, Søren.E.; Hansen, A.G.; Steffensen, R.; Jacobsen, C.; Jensenius, J.C.; Thiel, S.

Journal of Immunology 183(11): 7371-7378


ISSN/ISBN: 0022-1767
PMID: 19917686
DOI: 10.4049/jimmunol.0902388
Accession: 054205312

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Essential effector functions of innate immunity are mediated by complement activation initiated by soluble pattern recognition molecules: mannan-binding lectin (MBL) and the ficolins. We present a novel, phylogenetically conserved protein, MAp44, which is found in human serum at 1.4 microg/ml in Ca(2+)-dependent complexes with the soluble pattern recognition molecules. The affinity for MBL is in the nanomolar range (K(D) = 0.6 nM) as determined by surface plasmon resonance. The first eight exons of the gene for MAp44 encode four domains shared with MBL-associated serine protease (MASP)-1 and MASP-3 (CUB1-EGF-CUB2-CCP1), and a ninth exon encodes C-terminal 17 aa unique to MAp44. mRNA profiling in human tissues shows high expression in the heart. MAp44 competes with MASP-2 for binding to MBL and ficolins, resulting in inhibition of complement activation. Our results add a novel mechanism to those known to control the innate immune system.

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