title: Complement C4b-binding protein as a novel murine epididymal secretory protein.
abstract: Complement C4b-binding protein (C4BP) is a plasma protein synthesized in the liver and plays a regulatory role in the host defense complement system. We have previously reported that mRNAs of the C4BP alpha chain (C4BPalpha) are expressed at significant levels in the guinea pig and mouse epididymis in an androgen-dependent manner. Here, we analyze the murine C4bpa gene and show that epididymal and liver C4BPalpha mRNAs are generated from a single-copy gene and that the epididymal C4BPalpha mRNAs are transcribed from novel transcription start sites located approximately 100 base pairs downstream from those used in the liver. Furthermore, in an immunohistochemical study using rabbit anti-mouse C4BP antiserum, we demonstrated that C4BP is localized in the stereocilia and Golgi apparatus of the epididymal epithelial cells and the surfaces of spermatozoa in the lumen in the region from the distal caput to the cauda but not in the proximal caput region. Indirect immunofluorescence of the isolated spermatozoa demonstrated that C4BP is localized preferentially on the head region of the spermatozoa, and immunoelectron microscopy located C4BP on the plasma membrane and the outer acrosomal membrane. These results indicate that epididymal C4BP is synthesized in the epithelial cells and secreted into the lumen in a region-restricted manner and is taken up to the sperm membranes on passage through the epididymis. Many epididymal proteins are secreted from the epithelial cells in a region-specific and androgen-dependent manner and are considered to contribute to sperm maturation. Our findings suggest a novel function of C4BP as one such epididymal secretory protein.
authors.affiliation: Department of Biological Sciences, University of Tokyo, Tokyo 133-0033, Japan. nmayumi@biol.s.u-tokyo.ac.jp
authors.forename: Mayumi I
authors.initials: MI
authors.lastname: Nonaka
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Hishikawa
authors.forename: Nobuo
authors.initials: N
authors.lastname: Moriyama
authors.forename: Takehiko
authors.initials: T
authors.lastname: Koji
authors.forename: Ronald T
authors.initials: RT
authors.lastname: Ogata
authors.forename: Akihiko
authors.initials: A
authors.lastname: Kudo
authors.forename: Hayato
authors.initials: H
authors.lastname: Kawakami
authors.forename: Masaru
authors.initials: M
authors.lastname: Nonaka
mesh_terms: D000818:Animals; D001483:Base Sequence; D015152:Blotting, Northern; D015153:Blotting, Western; D003001:Cloning, Molecular; D004822:Epididymis / Q000378:metabolism*; D004847:Epithelial Cells / Q000378:metabolism; D005786:Gene Expression Regulation; D006649:Histocompatibility Antigens / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D008099:Liver / Q000502:physiology; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D013079:Sperm Maturation / Q000502:physiology; D013094:Spermatozoa / Q000378:metabolism; D024363:Transcription Initiation Site
chemical_list: C489166:C4bp protein, mouse; D006649:Histocompatibility Antigens