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. 1999 Sep 14;96(19):10794-9.
doi: 10.1073/pnas.96.19.10794.

Cloning of gp-340, a putative opsonin receptor for lung surfactant protein D

U Holmskov  1 , J Mollenhauer, J Madsen, L Vitved, J Gronlund, I Tornoe, A Kliem, K B Reid, A Poustka, K Skjodt
Affiliations

Cloning of gp-340, a putative opsonin receptor for lung surfactant protein D

U Holmskov et al. Proc Natl Acad Sci U S A. .

Abstract

Surfactant protein D (SP-D) is an oligomeric C type lectin that promotes phagocytosis by binding to microbial surface carbohydrates. A 340-kDa glycoprotein (gp-340) has been shown to bind SP-D in the presence of calcium but does so independently of carbohydrate recognition. This protein exists both in a soluble form and in association with the membranes of alveolar macrophages. The primary structure of gp-340 has been established by molecular cloning, which yielded a 7,686-bp cDNA sequence encoding a polypeptide chain of 2, 413 amino acids. The domain organization features 13 scavenger receptor cysteine-rich (SRCR) domains, each separated by an SRCR-interspersed domain, except for SRCRs 4 and 5, which are contiguous. The 13 SRCR domains are followed by two C1r/C1s Uegf Bmp1 domains separated by a 14th SRCR domain and a zona pellucida domain. gp-340 seems to be an alternative spliced form of DMBT1. Reverse transcription-PCR analysis showed that the main sites of synthesis of gp-340 are lung, trachea, salivary gland, small intestine, and stomach. Immunohistochemistry revealed strong staining for gp-340 in alveolar and other tissue macrophages. Immunostaining of the macrophage membrane was either uniform or focal in a way that suggested capping, whereas other macrophages showed strong intracellular staining within the phagosome/phagolysosome compartments. In some macrophages, SP-D and gp-340 were located in the same cellular compartment. Immunoreactive gp-340 was also found in epithelial cells of the small intestine and in the ducts of salivary glands. The distribution of gp-340 in macrophages is compatible with a role as an opsonin receptor for SP-D.

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Figures

Figure 1
Figure 1
Structure of gp-340. The cDNA clones used to establish the sequence of gp-340 are shown together with a schematic representation of the domain structure of gp-340 drawn to scale. The positions of the cDNA clones and long-range PCR (DMBT1/6-kb and DMBT1/8-kb.2) products are indicated as solid lines. SID, SRCR-interspersed domain.
Figure 2
Figure 2
The deduced amino acid sequence of gp-340. SRCR domains are shown in blue, SIDs in yellow, CUB domains in green, Ser-Pro-Thr-rich regions in orange, and the ZP domain in pink. The hydrophobic putative signal peptide is underlined. Potential glycosylation sites are shown in boxes.
Figure 3
Figure 3
Alignment of gp-340 domains and domain organization of gp-340 compared with other members of the SRCR superfamily. (A) Alignment of the SRCR domains of gp-340. (B) Alignment of the SIDs of gp-340. (C) Alignment of the CUB domains of gp-340. (D) Alignment of the C-terminal part of gp-340 with the same region of CRP-ductins and Ebnerin. The transmembrane (Tm) region was obtained from a genomic DMBT1 clone containing a putative exon for this region. (E) The domain organization of gp-340 is compared with the 6.0-kb form of DMBT1 and other members of group B of the SRCR superfamily that contain SRCR and ZP domains.
Figure 4
Figure 4
RT-PCR analysis of gp-340/DMBT1 expression in human tissues. The Southern blots were exposed for 1 h for all tissues or for 48 h omitting tissues 1, 5, 7, 15, and 16 to avoid overexposure.
Figure 5
Figure 5
Immunohistochemistry of gp-340 in normal human tissues with monoclonal (a–g, i, and j) or polyclonal (k) antibody directed against gp-340. (a–f) Sections of human alveolar macrophages, obtained by cytospin and stained by the monoclonal antibody directed against gp-340. (g and h) Serial sections of human lung stained with monoclonal antibodies directed against gp-340 (g) or SP-D (h). (i–k) Immunoreactive gp-340 was also found in salivary ducts (i), in certain cells of the pancreatic islets (j), and in epithelial cells of the small intestine (k).
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