CRP-ductin, the mouse homologue of gp-340/deleted in malignant brain tumors 1 (DMBT1), binds gram-positive and gram-negative bacteria and interacts with lung surfactant protein D

CRP-ductin is a protein expressed mainly by mucosal epithelial cells in the mouse. Sequence homologies indicate that CRP-ductin is the mouse homologue of human gp-340, a glycoprotein that agglutinates microorganisms and binds the lung mucosal collectin surfactant protein-D (SP-D). Here we report that purified CRP-ductin binds human SP-D in a calcium-dependent manner and that the binding is not inhibited by maltose. The same properties have previously been observed for gp-340 binding of SP-D. CRP-ductin also showed calcium-dependent binding to both gram-positive and -negative bacteria. A polyclonal antibody raised against gp-340 reacted specifically with CRP-ductin in Western blots. Immunoreactivity to CRP-ductin was found in the exocrine pancreas, in epithelial cells throughout the gastrointestinal tract and in the parotid ducts. A panel of RNA preparations from mouse tissues was screened for CRP-ductin and SP-D expression by reverse transcription-PCR. The pancreas was the main site of synthesis of CRP-ductin, but transcripts were also readily amplified from salivary gland, the gastrointestinal tract, liver, testis, uterus and lung. Lung was the main site of synthesis of SP-D, but transcripts were also amplified from uterus, salivary gland, thymus, thyroid gland, pancreas and testis. We conclude that CRP-ductin is the mouse homologue of human gp-340 and that its capacity to bind SP-D as well as gram-negative and gram-positive bacteria suggests a role in mucosal immune defense.