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BOR - Papers in Press, published online ahead of print August 20, 2003.
Biol Reprod 2003, 10.1095/biolreprod.103.020289
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BIOLOGY OF REPRODUCTION 69, 1931–1939 (2003)
DOI: 10.1095/biolreprod.103.020289
© 2003 by the Society for the Study of Reproduction, Inc.

Immunology

Complement C4b-Binding Protein as a Novel Murine Epididymal Secretory Protein

Mayumi I. Nonaka1,2, Yoshitaka Hishikawa3, Nobuo Moriyama4, Takehiko Koji3, Ronald T. Ogata5, Akihiko Kudo6, Hayato Kawakami6, and Masaru Nonaka2

Department of Biological Sciences,2 University of Tokyo, Tokyo 133-0033, Japan Department of Histology and Cell Biology,3 Nagasaki University School of Medicine, Nagasaki 852-8523, Japan Department of Experimental Nursing,4 Faculty of Nursing, Fukuoka Prefectural University, Tagawa 825-8585, Japan Torry Pines Institute for Molecular Studies,5 San Diego, California 92121 Department of Anatomy, Kyorin University School of Medicine,6 Mitaka, Tokyo 181-8611, Japan

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 (C4BP{alpha}) 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 C4BP{alpha} mRNAs are generated from a single-copy gene and that the epididymal C4BP{alpha} 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.

1 Correspondence: Mayumi I. Nonaka, Department of Biological Sciences, University of Tokyo, Hongo 7-3-1, Tokyo, 113-0033 Japan. FAX: 81-3-5800-3397; nmayumi{at}biol.s.u-tokyo.ac.jp






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