CD300 antigen like family member G: A novel Ig receptor like protein exclusively expressed on capillary endothelium Hiroyuki Takatsu a,b , Koji Hase a , Masumi Ohmae a , Sayaka Ohshima a , Koji Hashimoto a , Naoko Taniura c , Akitsugu Yamamoto d , Hiroshi Ohno a,b, * a Laboratory for Epithelial Immunobiology, Research Center for Allergy and Immunology, RIKEN, Yokohama 230-0045, Japan b Supramolecular Biology, International Graduate School of Arts and Sciences, Yokohama City University, Yokohama, Japan c Division of Cell Cycle Regulation, Cancer Research Institute, Kanazawa University, 13-1 Takara-machi, Kanazawa, Ishikawa 920-0934, Japan d Nagahama Institute of Bio-Science and Technology, Shiga 526-0829, Japan Received 3 July 2006 Available online 21 July 2006 Abstract We report the characteristics of CD300LG, a member of the CD300 antigen like family. Its genomic structure is similar in both mouse and human, and at least four isoforms exist in both species. The amino acid sequence of the immunoglobulin (Ig) V like domain of CD300LG showed approximately 35% identity to those of the polymeric Ig receptor (pIgR) and Fca/lR. Interestingly, mouse CD300LG proteins were uniquely expressed on capillary endothelium. Immunoelectron microscopy revealed that mouse CD300LG is localized on both apical and basolateral plasma membranes, as well as on intracellular vesicular structures, in the capillary endothelium. Transcytosis assays using polarized MDCK epithelial cells showed that CD300LG could be transcytosed bidirectionally. Furthermore, CD300LG exogenously expressed on HeLa cells could take up IgA2 and IgM, but not IgG. These results suggest that CD300LG might play an important role in molecular traffic across the capillary endothelium. Ó 2006 Elsevier Inc. All rights reserved. Keywords: CD300L; CMRF35; Immunoglobulin super family; Capillary endothelium; Transcytosis; IgA; IgM The Ig superfamily (IgSF) consists of a number of cell surface glycoproteins that share sequence homology with the V and C domains of antibody (Ab) heavy and light chains, and is involved in many important functions including leukocyte adhesion, antigen recognition, and activation and inhibition of immune response [1,2]. Cur- rently, several members of the IgSF are grouped into small- er families that have related genes or, alternatively, several isoforms of different molecules such as killer cell Ig like receptors [3], leukocyte Ig like receptors [4], Fc receptors [5], signal-regulatory proteins [6], triggering receptor expressed on myeloid cells (TREM) [7], paired Ig like type 2 receptor [8], and CD300L [9]. The CD300L family is widely found on most hemato- poietic cells including monocytes, neutrophils, macro- phages, dendritic cells, subpopulations of lymphocytes and bone marrow cells [9]. Each member of this family is a type I cell surface glycoprotein containing a single Ig V like domain. In the past GenBank database, some mouse genes of the CD300L family had been termed polymeric Ig receptor (pIgR) precursors because their Ig V like domains have significant sequence homologies to that of pIgR, the Fc receptor for polymeric IgA and IgM [10]. On the other hand, human genes of the CD300L family have been identified as an antigen to the CMRF35 mono- clonal antibody (mAb) [11]. Subsequently, it has been revealed that the human CMRF35 antigens are orthologs of mouse pIgR precursors. Recently, this family has been renamed CD300 antigen like (A–G) in both mouse and human. It has been demonstrated that the CD300L family 0006-291X/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2006.07.047 * Corresponding author. Fax: +81 45 503 7030. E-mail address: ohno@rcai.riken.jp (H. Ohno). www.elsevier.com/locate/ybbrc Biochemical and Biophysical Research Communications 348 (2006) 183–191 BBRC