Membrane-bound Versus Secreted Forms of Human Asialoglycoprotein Receptor Subunits

doi:10.1074/jbc.271.24.14496

Membrane-bound Versus Secreted Forms of Human Asialoglycoprotein Receptor Subunits

ROLE OF A JUXTAMEMBRANE PENTAPEPTIDE*

From the ^‡ Department of Cell Research and Immunology, George Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel, 69978,
^§ Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, and the
^∥ Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

^” To whom correspondence should be addressed. Tel.: 972-3-640-9239; Fax: 972-3-642-2046; E-mail: gerardo{at}post.tau.ac.il

↵^¶ Present address: Dept. of Microbiology and Immunology, UCLA School of Medicine, Center for the Health Sciences, Los Angeles CA, 90095.

Abstract

The H2a alternatively spliced variant of the human asialoglycoprotein receptor H2 subunit differs from the H2b variant by an extra pentapeptide, EGHRG, present in the ectodomain next to the membrane-span. This difference causes retention and degradation in the endoplasmic reticulum (ER) of H2a when expressed without the H1 subunit in 3T3 cells (1). In contrast, a significant portion of singly expressed H2b is Golgi-processed and reaches the cell surface. Using a new specific anti-H2a antibody, we found that in HepG2 cells, H2a is rapidly cleaved to a 35-kDa fragment, comprising the entire ectodomain, most of which is secreted into the medium. The cleavage site for the secreted fragment was located at the lumenal end of the membrane span. No membrane-bound H2a exits the ER, indicating that the pentapeptide is a signal for ER retention and degradation of the membrane form but does not hinder secretion of the cleaved soluble form. H2a does not form a membrane receptor complex with H1 as H2b does. H2a is therefore not a subunit of the receptor but a precursor for a secreted form of the protein; signal peptidase is probably responsible for the cleavage to the soluble fragment (2). Therefore, the juxtamembrane sequence regulates the function of the transmembrane domain of a type II membrane protein as either a signal-anchor sequence (H2b) or as a cleaved signal sequence, which generates a secreted product (H2a).

Footnotes

↵* This work was supported by a grant from the Israel Cancer Research Foundation and Grant 91–00–218 from the US-Israel Binational Science Foundation (both to G. Z. L.) and Grant GM-35012 from the National Institutes of Health (to H. F. L.) 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.
↵¹ The abbreviations used are:

ASGPR

asialoglycoprotein receptor

ER

endoplasmic reticulum

endo H

endo-N-acetylglucosaminidase H

PMSF

phenylmethylsulfonyl fluoride

PBS

phosphate-buffered saline

DMEM

Dulbecco's modified Eagle's medium

PAGE

polyacrylamide gel electrophoresis.
↵² S. Tolchinsky and G. Z. Lederkremer, unpublished results.
- Received February 29, 1996.