Plant Molecular Biology 34: 507–515, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Interaction of rice and human SRP19 polypeptides with signal recognition
Kimberly Chittenden, Krishne Gowda, Shaun D. Black and Christian Zwieb
Departments of Molecular Biology and Biochemistry, The University of Texas Health Science Center,
P.O. Box 2003, Tyler, TX 75710, USA (
author for correspondence)
Received 20 January 1997; accepted in revised form 4 April 1997
Key words: comparative sequence analysis, protein folding, protein targeting, ribonucleoprotein particles, RNA
protein interactions, RNA secondary structure
The signal recognition particle (SRP) controls the transport of secretory proteins into and across lipid bilayers.
SRP-like ribonucleoproteincomplexes exist in all organisms, including plants. We characterized the rice SRP RNA
and its primary RNA binding protein, SRP19. The secondary structure of the rice SRP RNA was similar to that
found in other eukaryotes; however, as in other plant SRP RNAs, a GUUUCA hexamer sequence replaced the
highly conserved GNRA-tetranucleotide loop motif at the apex of helix 8. The small domain of the rice SRP RNA
was reduced considerably. Structurally, rice SRP19 lacked two small regions that can be present in other SRP19
homologues.Conservativestructureprediction and site-directedmutagenesisof riceand human SRP19 polypeptides
indicated that binding to the SRP RNAs occurred via a loop that is present in the N-domain of both proteins. Rice
SRP19 protein was able to form a stable complex with the rice SRP RNA in vitro. Furthermore, heterologous
ribonucleoprotein complexes with components of the human SRP were assembled, thus conﬁrming a high degree
of structural and functional conservation between plant and mammalian SRP components.
The signal recognition particle (SRP) is a cytoso-
lic ribonucleoprotein complex which is part of the
machinery that targets secretory proteins to cellular
membranes [19, 34]. In vitro, wheat and maize SRPs
promoted the association between the ER membrane
and ribosomes that contained nascent polypetides [6,
23]. An extensive cultivar- and tissue-independent
sequence variability that is unique to plant SRP RNAs,
was instrumental in deciphering common RNA sec-
ondary structure elements [15, 16]. As a general rule,
the SRP RNAs of the archaea and the higher euka-
ryotes, including plant, are about 300 nucleotides long
and fold into similar secondary structures.
The sizes of the RNA molecules may vary con-
siderably in the lower eukaryotes and in the bacteria
The nucleotidesequencedatareported will appear in the EMBL,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession numbers U19030 and U85037.
(e.g. 519 bases in the yeast Saccharomyces cerevisiae,
77 bases in the bacterium Mycoplasma mycoides) .
All SRP RNAs share RNA helix 8 and an associated
binding-protein, called SRP54 or ffh (P48) (see [16,
19] for reviews). This suggests an important role for
the SRP in all organisms.
Besides SRP54, there are ﬁve additional proteins
(SRP9, SRP14, SRP19, SRP68, and SRP72) present
in the mammalian SRP . Although a similar set
of SRP proteins would be predicted to exist in all
plants, we know only about plant SRP54 homologues
from Arabidopsis  and from barley (Chu et al.,
unpublished, accession numbers P49968, P49969, and
P49970), and about the ffh protein from Arabidopsis
chloroplast . The presented characterization of the
rice SRP19 protein is the ﬁrst report of a SRP19 homo-
logue from a plant.
Since it is the only protein capable of binding to
the SRP RNA in the absence of other macromolecu-
lar components , SRP19 is considered to be the
GR: 201001956, Pips nr. 138688 BIO2KAP
pla427us.tex; 5/06/1997; 8:22; v.7; p.1