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

Surfactant protein D (SP-D) is an oligomeric C type lectin that promotes ph agocytosis by binding to microbial surface carbohydrates. A 340-kD glycopro tein (gp-340) has been shown to bind SP-D in the presence of calcium but do es so independently of carbohydrate recognition, This protein exists both i n a soluble form and in association with the membranes of alveolar macropha ges, The primary structure of gp-340 has been established by molecular clon ing, 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 f ollowed by two C1r/C1s Uegf Bmp1 domains separated by a 14th SRCR domain an d a zona pellucida domain, gp-340 seems to be an alternative spliced form o f DMBT1. Reverse transcription-PCR analysis showed that the main sites of s ynthesis of gp-340 are lung, trachea, salivary gland,small intestine, and s tomach. Immunohistochemistry revealed strong staining for gp-340 in alveola r and other tissue macrophages. Immunostaining of the macrophage membrane w as either uniform or focal in a way that suggested capping, whereas other m acrophages showed strong intracellular staining within the phagosome/phagol ysosome compartments. In some macrophages, SP-D and gp-340 were located in the same cellular compartment, Immunoreactive gp-340 was also found in epit helial cells of the small intestine and in the ducts of salivary glands. Th e distribution of gp-340 in macrophages is compatible with a role as an ops onin receptor for SP-D.