Plant Molecular Biology 33: 1085–1092, 1997.
1085
c
1997 Kluwer Academic Publishers. Printed in Belgium.
Short communication
Cloning and characterization of the OsNramp family from Oryza sativa,a
new family of membrane proteins possibly implicated in the transport of
metal ions
Abdelmajid Belouchi, Tony Kwan and Philippe Gros
Department of Biochemistry, McGill University, Montr
´
eal Qu
´
ebec, Canada, H3G-1Y6 (
author for
correspondence)
Received 17 June 1996; accepted in revised form 24 December 1996
Key words: gene family, rice, tissue-specific gene expression, membrane protein
Abstract
The mammalian Nramp1 protein is an integral membrane protein expressed exclusively in macrophages, where
it plays a critical role in the ability of these cells to destroy ingested microbes. The bactericidal mechanism of
action of Nramp1 remains unknown. We report the identification and characterization of cDNA clones corres-
ponding to three homologues of the mammalian Nramp1 gene from the genome of Oryza sativa, OsNramp1,
OsNramp2,andOsNramp3. These three genes encode a novel group of highly similar hydrophobic polypeptides
sharing between64% and 75% sequence similarity, that show similar hydropathyprofiles, and predicted secondary
structure, including the same number, position, and sequence characteristics (including conserved charges) of
transmembrane domains. Together, these define a highly conserved membrane associated hydrophobic core. The
three plant proteins show a remarkable degree of sequence similarity with their mammalian counterpart (60% to
70% similarity), including primary and secondary structure elements previously described in ion transporters and
channels. Expression studies in normal plant tissues indicate that while OsNramp1 is expressed primarily in roots,
and OsNramp2 is primarily expressed in leaves, OsNramp3 is expressed in both tissues. The recent discovery
that the yeast Nramp homologue SMF1 functions as a manganese transporter raises the exciting possibility that
OsNramp encodes a family of metal ion transporters in plants.
In mice, resistance or susceptibility to infection with
intracellular microbes is affected by mutations at the
Bcg locus [14]. This gene controls the ability of mac-
rophages and monocytes to restrict the intracellular
proliferation of antigenically unrelated parasites [16].
Bcg [17, 30] codes for a novel integral membrane pro-
tein (Nramp1 [17]) that shares structural characterist-
ics with several transporters and channels [5], includ-
ing twelve transmembrane (TM) domains, a glyc-
osylated loop (between predicted TM6 and TM7), and
a sequence signature (in the TM8 to TM9 interval)
[1] that shares similarity to the permeation pore of the
K
+
channel family [19], and the proteolipid subunit of
vacuolarH
+
ATPases[6].Nramp1isexpressedexclus-
ively in phagocytes, and we have proposed that its
bacteriostatic or bactericidal activity may be through
modulation of the electrolyte content of the phago-
some [16, 17], the vesicular compartment in which
intracellular microbes reside [10]. Nramp is a gene
family highly conserved throughout evolution [4, 5].
Members of this family include bovine, ovine and
avian homologues (P. Gros, unpublished), the mlv
gene of the fly Drosophila melanogaster (GenBank,
accession number U23948) in which mutations cause
a loss of neuronal sensory function [12], two homo-
logues in the worm Caenorhabditis elegans(GenBank
U23525), three genes (SMF1, SMF2, and an anonym-
ous sequence; GenBank U15929, U00062, U34585)
in the yeast Saccharomyces cerevisiae [18] in which
mutations cause hypersensitivity to metal chelators
GR: 201001874, Pips nr. 131319 BIO2KAP
*131319*
pl329us.tex; 2/04/1997; 7:40; v.7; p.1