The colonic epithelium-enriched protein, a4, is a proteolipid that exhibits ion channel characteristics. Breitwieser, Gerda E., John C. McLenithan, Joseph F. Cortese, Janiel M. Shields, Maria M. Oliva, Jessica L. Majewski, Carolyn E. Machamer, and Vincent W. Yang. Departments of Medicine, Physiology, Biological Chemistry, Pediatrics, and Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
APStracts 3:0296C, 1996.
Expression of the human gene A4 is enriched in the colonic epithelium and is transcriptionally activated upon differentiation of colonic epithelial cells in vitro [Oliva et al. (1993) Arch. Biochem. Biophys. 302: 183-192]. A4 cDNA contains an open reading frame that predicts a polypeptide of 17 kDa MW. To determine the function of the A4 protein, we characterized its biochemical and physiological properties. Hydropathy analysis of deduced A4 amino acid sequence revealed four putative membrane-spanning [alpha] helices. The hydrophobic nature of A4 was confirmed by its being extractable with organic solvents. Immunocytochemical studies of cells expressing A4 localized it to the endoplasmic reticulum. Moreover, A4 multimerized in vivo as determined by coimmunoprecipitation experiments. The four -transmembrane topology and biophysical characteristics of A4 suggest that it belongs to a family of integral membrane proteins called proteolipids, some of which multimerize to form ion channels. Subsequent electrophysiological studies of nuclei isolated from microinjected Xenopus laevis oocytes transiently expressing A4 showed the appearance of a 28 pS channel. Thus, our studies indicate that A4 is a colonic epithelium-enriched protein localized to the endoplasmic reticulum and that, similar to other proteolipids, A4 multimerizes and exhibits characteristics of an ion channel. 

Received 18 July 1996; accepted in final form 9 September 1996.
APS Manuscript Number C401-6.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996