Mitochondrial transcription factor A (TFAM), a transcription factor for mitochondrial DNA (mtDNA) that also possesses the property of nonspecific DNA binding, is essential for maintenance of mtDNA. To clarify the role of TFAM, we repressed the expression of endogenous TFAM in HeLa cells by RNA interference. The amount of TFAM decreased maximally to about 15% of the normal level at day 3 after RNA interference and then recovered gradually. The amount of mtDNA changed closely in parallel with the daily change in TFAM while in organello transcription of mtDNA at day 3 was maintained at about 50% of the normal level. TFAM lacking its C-terminal 25 amino acids (TFAM-⌬C) marginally activated transcription in vitro. When TFAM-⌬C was expressed at levels comparable to those of endogenous TFAM in HeLa cells, mtDNA increased twofold, suggesting that TFAM-⌬C is as competent in maintaining mtDNA as endogenous TFAM under these conditions. The in organello transcription of TFAM-⌬C-expressing cells was no more than that in the control. Thus, the mtDNA amount is finely correlated with the amount of TFAM but not with the transcription level. We discuss an architectural role for TFAM in the maintenance of mtDNA in addition to its role in transcription activation.

Collectins are a family of C-type lectins that have collagen-like sequences and carbohydrate recognition domains (CRD). They are involved in host defense through their ability to bind to carbohydrate antigens of microorganisms. The scavenger receptors type A and MARCO are classical type scavenger receptors that have internal collagen-like domains. Here we describe a new scavenger receptor that is a membrane-type collectin from placenta (collectin placenta 1 (CL-P1)), which has a typical collectin collagen-like domain and a CRD. The cDNA has an insert of about 2.2 kilobases coding for a protein containing 742 amino acid residues. The deduced amino acid sequence shows that CL-P1 is a type II membrane protein, has a coiled-coil region, a collagen-like domain, and a CRD. It resembles type A scavenger receptors because the scavenger receptor cysteine-rich domain is replaced by a CRD. Northern analyses, reverse transcription-polymerase chain reaction, and immunohistochemistry show that CL-P1 is expressed in vascular endothelial cells but not in macrophages. By immunoblotting and flow cytometry CL-P1 appears to be a membrane glycoprotein of about 140 kDa in human umbilical vein or arterial endothelial cells, placental membrane extracts, and CL-P1 transfected Chinese hamster ovary cells. We found that CL-P1 can bind and phagocytose not only bacteria (Escherichia coli and Staphylococcus aureus) but also yeast (Saccharomyces cerevisiae). Furthermore, it reacts with oxidized low density lipoprotein (OxLDL) but not with acetylated LDL (AcLDL). These binding activities are inhibited by polyanionic ligands (polyinosinic acid, polyguanylic acid, dextran sulfate) and OxLDL but not by polycationic ligands (polyadenylic acid or polycytidylic acid), LDL, or AcLDL. These results indicate that CL-P1 might play important roles in host defenses that are different from those of soluble collectins in innate immunity.

Collectins are a family of C‐type lectins with two characteristic structures, collagen like domains and carbohydrate recognition domains. They recognize carbohydrate antigens on microorganisms and act as host‐defense. Here we report the cloning and characterization of a novel collectin CL‐K1. RT‐PCR analyses showed CL‐K1 mRNA is present in all organs. The deduced amino acid sequence and the data from immunostaining of CL‐K1 cDNA expressing CHO cells revealed that CL‐K1 is expressed as a secreted protein. CL‐K1 is found in blood by immunoblotting and partial amino acid analyses. CL‐K1 showed Ca2+‐dependent sugar binding activity of fucose and weakly mannose but not N‐acetyl‐galactosamine, N‐acetyl‐glucosamine, or maltose, though mannose‐binding lectin (MBL) containing similar amino acid motif. CL‐K1 can recognize specially several bacterial saccharides due to specific sugar‐binding character. Elucidation of the role of two ancestor collectins of CL‐K1 and CL‐L1 could lead to see the biological function of collectin family.

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