Characterization of p18, a Component of the Lamin B Receptor Complex and a New Integral Membrane Protein of the Avian Erythrocyte Nuclear Envelope

doi:10.1074/jbc.271.21.12617

Characterization of p18, a Component of the Lamin B Receptor Complex and a New Integral Membrane Protein of the Avian Erythrocyte Nuclear Envelope (*)

From the (1)Program of Cell Biology, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Federal Republic of Germany and the
(2)Department of Basic Sciences, Faculty of Medicine, The University of Crete, Stavrakia, 711 10 Heraklion, Crete, Greece

^¶¶ To whom correspondence should be addressed. Tel.: 30-81-542-070 (ext. 226); Fax: 30-81-542-112.

↵** Present address: Universitadät Heidelberg, Institut für Biochemie I, Im Neuenheimer Feld 328, D-69120 Heidelberg, Germany.

Abstract

Employing avian erythrocytes, we have previously isolated a multimeric complex consisting of the lamin B receptor (LBR, or p58), the nuclear lamins, an LBR-specific kinase, a 34-kDa protein, and an 18-kDa polypeptide termed p18. As the LBR kinase and the 34-kDa component have been recently characterized, we now proceed in the characterization of p18. We show here that p18 is an integral membrane protein specific to the erythrocyte nuclear envelope which binds to LBR and B-type lamins. NH-terminal sequencing indicates that p18 is distinct from other nuclear envelope components, but has similarity to the mitochondrial isoquinoline-binding protein. In situ analysis by immunoelectron microscopy and examination of digitonin-permeabilized cells by indirect immunofluorescence show that p18, unlike LBR and other lamin-binding proteins, is equally distributed between the inner and outer nuclear membrane. Furthermore, cycloheximide inhibition experiments reveal that the fraction of p18 that resides in the outer nuclear membrane does not represent nascent chains en route to the inner nuclear membrane, but rather material in equilibrium with the p18 that partitions with the inner nuclear membrane. The paradigm of p18 suggests that transmembrane complexes formed by the nuclear lamins and LBR provide potential docking sites for integral membrane proteins of the nuclear envelope that equilibrate between the rough endoplasmic reticulum and the inner nuclear membrane.

Footnotes

↵^§ Supported by a fellowship from the BIOMED I program of the European Union.
↵^¶ These authors have contributed equally to this work.
↵^§§ Supported by the Biotechnology Program of the European Union.
↵* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This work is dedicated to Stavros and Adamantia Politis.
↵() The abbreviations used are:

LBR

lamin B receptor

LAP

lamina-associated polypeptide

PMSF

phenylmethylsulfonyl fluoride

DTT

dithiothreitol

PAGE

polyacrylamide gel electrophoresis

IBP

isoquinoline-binding protein

PBS

phosphate-buffered saline

TM

transmembrane domain.
- Received December 26, 1995.
- Revision received February 14, 1996.