Membrane Cofactor Protein (CD46) Is a Basolateral Protein That Is Not Endocytosed
IMPORTANCE OF THE TETRAPEPTIDE FTSL AT THE CARBOXYL TERMINUS*
- Andrea Maisner‡,
- Gert Zimmer‡,
- M. Kathryn Liszewski§,
- Douglas M. Lublin¶,
- John P. Atkinson§ and
- Georg Herrler‡‖
- From the ‡Institut für Virologie, Philipps-Universität Marburg, D-35037 Marburg, Germany, the§Division of Rheumatology, Department of Internal Medicine, and the ¶Division of Laboratory Medicine, Department of Pathology and Medicine, Washington University School of Medicine, St. Louis, Missouri 63110
Abstract
Membrane cofactor protein (MCP) is a widely distributed complement regulatory protein that is expressed on the basolateral surface of polarized epithelial cells. The basolateral targeting of the BC1 isoform of MCP was analyzed by generating deletion mutants and point mutants within the cytoplasmic tail of 16 amino acids. A sequence of four amino acids, FTSL, was found to be indispensable for the basolateral transport of MCP. This tetrapeptide has two unique features compared with the targeting motifs of other basolateral proteins: (i) it contains a phenylalanine rather than a tyrosine at position 1; (ii) it is located at the very COOH-terminal end. Replacement of the phenylalanine or the leucine by an alanine resulted in a nonpolarized delivery to the cell surface. On the other hand, substitution of a tyrosine for the phenylalanine did not affect the basolateral transport of MCP. The latter mutant, however, was efficiently internalized, whereas the wild type protein was not subject to endocytosis. Our results indicate that the targeting signal YXX-large aliphatic that is involved in various sorting events has been modulated in MCP in such a way that it allows basolateral transport but not endocytosis.
Footnotes
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↵* This work was supported by Deutsche Forschungsgemeinschaft Grant He 1168/3-3 (to G. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‖ To whom correspondence should be addressed: Institut für Virologie, Philipps-Universität Marburg, Robert-Koch Strasse 17, D-35037 Marburg, Germany. Tel.: 49-6421-28-5360; Fax: 49-6421-28-5482. E-mail: herrler{at}mailer.uni-marburg.de.
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↵1 The abbreviations used are: GPI, glycosylphosphatidylinositol; MCP, membrane cofactor protein; SCR, short consensus repeat; STP, serine, threonine, proline-rich region; MDCK, Madin-Darby canine kidney; DAF, decay accelerating factor; TM, transmembrane; mAb, monoclonal antibody; FITC, fluorescein isothiocyanate; PBS, phosphate-buffered saline; VCNA, Vibrio cholerae sialidase; wt, wild type; SA, sialic acid(s).
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- Received April 30, 1997.
