Crystal Structure of the Cysteine-rich Domain of Scavenger Receptor MARCO Reveals the Presence of a Basic and an Acidic Cluster That Both Contribute to Ligand Recognition

Abstract

MARCO is a trimeric class A scavenger receptor of macrophages and dendritic cells that recognizes polyanionic particles and pathogens. The distal, scavenger receptor cysteine-rich (SRCR) domain of the extracellular part of this receptor has been implicated in ligand binding. To provide a structural basis for understanding the ligand-binding mechanisms of MARCO, we have determined the crystal structure of the mouse MARCO SRCR domain. The recombinant SRCR domain purified as monomeric and dimeric forms, and their structures were determined at 1.78 and 1.77 Å resolution, respectively. The monomer has a compact globular fold with a twisted five-stranded antiparallel ␤-sheet and a long loop covering a single ␣-helix, whereas the dimer is formed via ␤-strand swapping of two monomers, thus containing a large eight-stranded ␤-sheet. Calculation of the surface electrostatic potential revealed that the ␤-sheet region with several arginines forms a basic cluster. Unexpectedly, an acidic cluster was found in the long loop region. In the monomer, the acidic cluster is involved in metal ion binding. Studies with cells expressing various SRCR domain mutants showed that all of the arginines of the basic cluster are involved in ligand binding, suggesting a cooperative binding mechanism. Ligand binding is also dependent on the acidic cluster and Ca 2؉ ions whose depletion appears to affect ligand binding at least by modulating the electrostatic potential or relative domain orientation. We propose that the SRCR domain dimerization can contribute to the recognition of large ligands by providing a means for the MARCO receptor oligomerization.MARCO belongs to a diverse group of scavenger receptors (SRs) 3 expressed by macrophages, dendritic cells, and certain endothelial cells (1). These germ line-encoded receptors, also known as pattern recognition receptors due to their ability to recognize conserved pathogen-associated molecular patterns, are considered as an important part of innate immunity, the evolutionarily conserved, first line host defense mechanism. In addition to pathogen-associated molecular patterns, a long list of SR ligands, often polyanionic in nature, includes pollution particles, polyribonucleotides, bacterial lipopolysaccharides, modified host molecules such as oxidized low density lipoprotein (LDL), and unmodified endogenous proteins (1, 2). The SRs are classified into several subgroups, of which class A SRs have primarily been associated with innate immunity. This class consists of five members: SR-A (SR-AI, -II, and -III/ SCARA1) (3, 4), MARCO (macrophage receptor with collagenous domain)/SCARA2 (5), CSR1 (cellular stress response 1) and CSR2/SCARA3 (6), SRCL (scavenger receptor with C-type lectin) I and II/SCARA4 (7, 8), and Tesr (testis-expressed scavenger receptor)/SCARA5 (class A scavenger receptor 5) (9, 10). All of these are trimeric type II membrane proteins with a similar predicted tertiary structure consisting of a short intracellular domain, a transmembrane domain, and a large extracellular domain with...

Silica binding and toxicity in alveolar macrophages

“…al. propose that multiple MARCO receptors can group together on the surface of the cell allowing the SRCR region to dimerize or oligomerize creating a much larger binding surface area, that is capable of binding physically large ligands such as silica and bacteria [52]. A schematic of the SR and the proposed silica binding sites can be found in Figure 2.…”

Sprouty proteins: modified modulators, matchmakers or missing links?

“…By contrast, the CRDs of other proteins play varying roles: the CRD of the calcium-sensing receptor (hCaR) is necessary for signal transmission from the N-terminal venus-flytrap domain to the seven-transmembrane domain (Hu et al 2001, Tan et al 2003; the mannose receptor uses the CRD as a carbohydrate recognition motif (Fiete et al 1998); the CRD from a rubella virus non-structural protein is essential for viral protease activity and virus replication (Zhou et al 2009); the scavenger receptor MARCO utilizes the CRD for ligand recognition (Ojalo et al 2007); and the disintegrinlike/CRD of ADAM12 functions as a cell adhesion domain (Zhao et al 2004).…”

Molecular and Functional Characterization of Mouse S5D-SRCRB: A New Group B Member of the Scavenger Receptor Cysteine-Rich Superfamily

“…The presence of short Pro, Ser, and Thr (PST)-rich polypeptides interspaced with contiguous SRCR domains is also frequently observed among SRCR-SF members. Available three-dimensional structures obtained from crystallization experiments indicate that group A and B SRCR domains present a highly conserved and compact core folding (a curved six-stranded b sheet cradling an a helix) with variable outer loop regions, likely giving rise to functional diversity (8)(9)(10)(11)(12)(13).…”