Protein Trafficking in Plant Cells

340 Pages·1998·17.481 MB·English

by  Gad Galili, Champa Sengupta-Gopalan, Aldo Ceriotti (auth.), Jürgen Soll (eds.)

PROTEIN TRAFFICKING IN PLANT CELLS
PROTEIN TRAFFICKING IN PLANT CELLS
Edited by
JÜRGENSOLL
Reprinted from Plant Molecular Biology, Vo1ume 38 (1-2),1998
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
Library of Congress Cataloging-in-Publication Data
ISBN 978-94-010-6229-9 ISBN 978-94-011-5298-3 (eBook)
DOI 10.1007/978-94-011-5298-3
Printed on acid-free paper
All Rights Reserved
©
1998 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1998
Softcover reprint ofthe hardcover 1st edition 1998
No part of the material protected by this copyright notice
may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying,
recording or by any information storage and retrieval
system, without written permission from the copyright
owner.
PLANT MOLECULAR BIOLOGY
Val. 38, Nos. 1-2 (September I, 11) 1998
Special issue:
Protein Trafficking in Plant Cells
Guest editor:
J. Soll
Foreword vii
The endoplasmic reticulum of plant cells and its role in protein maturation and biogen-
esis of oil bodies
G. Galili, C. Sengupta-Gopalan, A Ceriotti 1-29
N-Glycoprotein biosynthesis in plants: recent developments and future trends
P. Lerouge, M. Ca banes- Macheteau, C. Rayon, A-C. Fischette-Laine, V. Gomord, L. Faye 31-48
The molecular characterization of transport vesicles
O.G. Robinson, G. Hinz, S.E.H. Holstein 49-76
Deposition of storage proteins
K. Müntz 77-99
Compartment-specific accumulation of recombinant immunogloblins in plant cells: an
essential tool for antibody production and immunomodulation of physiological func-
tions and pathogen activity
U. Conrad, U. Fiedler 101-109
Exocytosis in plants
G. Thiel, N. Battey 111-125
Sorting of proteins to vacuoles in plant cells
J.-M. Neuhaus, J.C. Rogers 127-144
The nuclear pore complex
A Heese-Peck, N.v. Raikhel 145-162
The surprising complexity of peroxisome biogenesis
L.J.Olsen 163-189
Protein translocation into and acr oss the chloroplastic envelope membranes
J. Soll, R. Tien 191-207
Multiple pathways for the targeting of thylakoid proteins in chloroplasts
C. Robinson, P.J. Hynds, O. Robinson, AMant 209-221
The role of lipids in plastid protein transport
B.O. Bruce 223-246
Protein import into cyanelles and complex chloroplasts
S.O. Schwartzbach, T. Osafune, W. Löffelhardt 247-263
Two birds with one stone: genes that encode products targeted to two or more com
partments
I. SmalI, H. Wintz, K. Akashi, H. Mireau 265-277
Intercellular protein trafficking through plasmodesmata
B. Ding 279-310
Mitochondrial protein import in plants
E. Glazer, S. Sjöling, M. Tanuaji, J. Whelan 311-338
Subject Index
339-341
tt
Plant Molecular Biology 38: vii, 1998.
Vll
Foreword
The proper protein complement for each subcellular compartment forms the basis for fhe functional complexity and
success of the eukaryotic cell. Except for some organellar proteins of mitochondria and plastids, all polypeptides
are coded far on nuclear genes synthesised in the cytosol and either co- or post-translationally transferred to their
final cellular destination. Questions of fundamental importance are: which signals are responsible for sorting and
targeting to a specific compartment and how is the transport through membranes or by vesicle ftow accomplished?
While we can recognize the signals responsible for subcellular sorting we just start to understand how translocation
across membranes or by vesicles ftow occurs. Knowledge of these processes is important since malfunctioning can
result in severe distortion of cellular function, for example the Zellweger syndrome in man. On the other hand,
plants convert 120 x 109 tonnes carbon dioxide into biomass annually, much of which is deposited as storage
proteins in grain and cereals we depend on for our diet. The potential of massif (large-scale) protein production
in plants not only adopted to special dietary needs for animals and man but also for the production of medical
diagnostics and pharmaceutics is now increasingly recognized. The perspective of plants as bioreactors to produce
foreign proteins of commercial interest seems to be almost unlimited. Many articles in this volume, therefore,
deal with the transport pathway and organelles involved in both protein ftux and storage, such as the endoplasmic
reticulum, Golgi network, the plant vacuole or plastid. Basic knowledge in this areas is essential in creating such
daring applications as to express immunglobulins in plants (Conrad, this volume).
The need for an up-to-date summary of our knowledge of intracellular protein trafficking is, therefore, evident
and pressing, since no recent comprehensive overview of the entire field exists. This volume is intended to fill this
gap and to survey all major areas of protein trafficking in plant cells. The information gathered here should be
valuable not only to the specialized plant researcher in the field and those working on applied aspects but also for
students and scientists working on protein translocation in non-plant systems. To meet the needs of a rapid moving
field such a special volume must be as up-to-date as possible to be useful. As the editor, I am indebted to the authors
and the reviewers who without hesitation agreed to meet a very tight time frame for their contribution. Therefore,
the reviewed literature goes weIl into 1998. Finally, I am thankful to the staff of Kluwer Academic Publishers,
particularly G. Jonkers and N. Bonnavalle, for their great interest in the assembly and timely publication of this
volume.
Kiel, Germany, April 1998
Plant Molecular Biology 38: 1-29, 1998.
© 1998 Kluwer Academic Puhlishers.
The endoplasmic reticulum of plant cells and its role in prot ein
maturation and biogenesis of oil bodies
Gad Galili 1,*, Champa Sengupta-Gopalan2 and Aldo Ceriotti3
I Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100 Israel (*author for correspon
dence); 2Agronomy and Horticulture Department, New Mexico State University Las Cruces, NM 88003, USA;
3Istituto Biosintesi Vegetali, Consiglio Nazionale delle Ricerche, Via Bassini 15, 20133, Milano, Italy
Key words: plants, endoplasmic reticulum, molecular chaperones, quality control, protein bodies, lipid bodies
Abstract
The endoplasmic reticulum (ER) is the port of entry of proteins into the endomembrane system, and it is also
involved in lipid biosynthesis and storage. This organelle contains a number of soluble and membrane-associated
enzymes and molecular chaperones, which assist the folding and maturation of proteins and the deposition of
lipid storage compounds. The regulation of translocation of proteins into the ER and their subsequent maturation
within the organelle have been studied in detail in mammalian and yeast cells, and more recently also in plants.
These studies showed that in general the functions of the ER in protein synthesis and maturation have been highly
conserved between the different organisms. Yet, the ER of plants possesses some additional functions not found in
mammalian and yeast cells. This compartment is involved in cell to cell communication via the plasmodesmata,
and, in specialized cells, it serves as a storage site for proteins. The plant ER is also equipped with enzymes and
structural proteins which are involved in the process of oil body biogenesis and lipid storage. In this review we
discuss the components of the plant ER and their function in protein maturation and biogenesis of oil bodies. Due
to the large number of cited papers, we were not able to cite all individual references and in many cases we refer
the readers to reviews and references therein. We apologize to the authors whose references are not cited.
Translocation of prot eins into the ER sequence, signal peptides are characterized by the
presence of three distinct regions: a positive charged
Signal sequences direct the targeting and n-region, followed by a hydrophobie h-region, and
translocation of soluble proteins into the ER apolar c-region that precedes the cleavage site. The
overall length of the signal peptide is determined by
The synthesis of proteins destined to cross the ER the number of amino acids that compose the n- and
membrane or to become integrated into it begins on h-regions, while the c-region shows very little length
free cytosolic ribosomes. Subsequent targeting and variation and is characterized by the presence of small
transport across the ER membrane can occur either co and neutral amino acids in position -3 and -1 rela
translationally or after the polypeptide chain has been tive to cleavage site [263]. It should however be noted
completed and released from the protein synthesizing that the n-region of some plant signal peptides does
apparatus. This lauer, post-translation al pathway has not strictly conform to the general rule, being in some
been most extensively characterized in yeast but is be cases neutral or even negatively charged [101]. Still,
lieved to operate to a variable extent in all eukaryotic on the basis of the features of known signal peptides,
cells [212]. useful methods have been developed which allow to
In soluble proteins, targeting to the ER membrane identify such targeting signals in novel proteins, and
is dictated by the presence of an N-terminal cleav to predict the site of cleavage when direct biochemical
able signal peptide. Although variable in length and evidence is not available [188].
2
Many proteins, including cytosolic pea albumin pore contained in a functional translocon has a diam
[1161, as weIl as bacterial [45, 47, 118, 200] and eter of 40-60 A [79] and is therefore much larger
synthetic [57] polypeptides, have been successfully than the minimum required to allow the passage of
targeted to the plant secretory system by placing vari a polypeptide chain in an extended conformation (5-
ous plant signal sequences at their N-terminus. These 7 A). Given the large size of the pore, a tight seal
and other examples show that addition of a signal pep between the ribosome and the translocon is likely to
tide is sufficient to achieve ER targeting of otherwise be important in maintaining the permeability barrier
cytosol-10cated proteins. between the ER lumen and the cytosol during co
An important question is whether all signal pep translational translocation [43], and protein fo1ding
tides can be considered functionally equivalent or might start even before the polypeptide appears on the
whether they require homologous components of the luminal site of the trans1ocon.
targeting, trans1ocation and processing machinery. In Homologues of the ß and y subunit of the Sec61
general, animal secretory proteins are correctly tar complex have been identified in plants [85], suggest
geted to the ER in plant cells, indicating that conserved ing a high degree of conservation of the machinery
features present in their signal peptides are correctly that drives protein translocation through the ER mem
recognized. For instance, when human serum albu brane.
min or immunoglobulin K chain were expressed in As mentioned above, a1though some proteins can
transgenic plants, their signal peptides were correctly be post-trans1ationally translocated also in mammalian
recognized and processed by the plant machinery, gen microsomes [131], the post-trans1ational mechanism
erating the correct N-terminus of the mature protein of protein import has been most extensive1y charac
[90, 238]. Some bacterial signal peptides may not be terized in Saccharamyces cerevisiae. This mechanism
equally functional in plants. When expressed in trans of insertion is SRP-independent, and is mediated by
genic tobacco, only a fraction of a secreted bacterial the heptameric Sec complex, (constituted the trimeric
chitinase was glycosylated, indicating inefficient seg Sec61 complex and four additional subunits), which
regation in the ER [161]. Substitution of the bacterial is sufficient for post-translational protein translocation
signal peptide with a plant one led to a dramatic in into reconstituted proteoliposomes [201]. In addition,
crease in glycosylation and secretion efficiency [160]. the viability of SRP-deficient cells clearly shows that
It should also be noted that a yeast signal peptide has the SRP-dependent pathway of pro tein targeting to the
been reported not to be functional in mammalian cells ER is not strictly required in S. cerevisiae [82].
[15]. In plants, ribosomes synthesizing soluble protein
In the cotranslational translocation mechanism precursors are found in association with the ER, and
(Figure 1), the signal peptide emerging from the ri a fraction of the ribosome-associated nascent chains
bosome is immediately recognized by a ribonucleo already lacks a signal peptide, indicating that cotrans-
protein particle, called the signal recognition particle 1ational import (and signal peptide cleavage) can occur
(SRP), and the complex composed of the ribosome, [17, 174, 227]. SRP has been partially character
the nascent protein chain and SRP is targeted to the ER ized in maize and wheat and has been shown to be
membrane [266]. Binding involves both an SRP re required for import of certain proteins into plant mi
ceptor (docking protein ) [172] and direct interactions crosomes [26, 210]. In the wheat germ translation
between the ribosome and proteins in the membrane system, the synthesis of nascent proteins bearing a sig
[123, 269]. This is followed by transfer of the signal nal sequence is blocked by canine SRP [267] and can
peptide from SRP to the translocation site. resurne only when docking protein or docking protein
The translocation process can be reconstituted in containing membranes are added [172]. In contrast,
vitra using proteoliposomes containing the SRP recep maize and wheat SRP do not stop the translation of
tor, the trimeric Sec61 complex and (for some protein signal peptide-containing proteins in the wheat germ
precursors) the translocating-chain associating mem translation system [26, 210], suggesting that the above
brane protein TRAM [77]. Altogether, these proteins mentioned translational block is due to the use of
constitute the minimal set of polypeptides sufficient components obtained from evolutionarily distant or
for protein import in vitra. The Sec61 complex is the ganisms. However, it is still possible that, as evidenced
key constituent of the actual translocation channel, in a mammalian system [273], a transient elongation
forming cylindrical oligomers within the ER mem arrest mayaiso be a characteristic of SRP-mediated
brane that can be directly visualized [81]. The aqueous targeting in plant cells.
3
• RP
4 3
SRP r~ceplor :pcfil coml>le~
Figure 1. A simplified model, depicting some of the events that accompany the targeting and translocation of a nascent protein chain to the ER
in mammals. Step I: the signal peptide of a nascent protein chain is recognized by a signal recognition protein (SRP), and a temary complex
constituted of the ribosome, the nascent polypeptide and SRP is formed. Step 2: the temary complex docks to the ER membrane, via interactions
with both the SRP receptor and the Sec61 complex. Step 3: the nascent protein chain is inserted into the translocon, which then opens toward
the ER, and both SRP and SRP-receptor are released. Step 4: the signal peptide is cleaved and the elongating protein chain is pushed into the ER
lumen. The reader is referred to recent reviews far a more camplete description of the targeting and translocation processes [6, 212]. Available
evidence suggest that the essential features of this model have been conserved in plants (see text for details).
Plant SRP differ from their mammalian counter these chaperones might be essential only in the case of
parts in containing an heterogeneous population of post-translationally translocated ones.
7SL RNA [27, 89, 165, 215], but the functional On the other side of the ER membrane, other
significance of this heterogeneity remains obscure chaperones have been shown to be involved in the
[215]. Plant homologues of two out of the six dif translocation process. Kar2p, the yeast homologue
ferent proteins that compose mammalian SRP have of the 78 kDa glucose-regulated protein (GRP78,
also been identified [32, 154]. Sequence compari BiP) is required for efficient post-translation al translo
son shows a high degree of evolutionary conservation, cation in reconstituted proteoliposomes containing
which is in keeping with the view that the process of yeast ER proteins [201], and a yeast strain carrying
SRP-mediated translocation in plants is fundamentally a temperature-sensitive Kar2 mutation is unable to
similar to the one that operates in other eukaryotic translocate pro tein precursor into the ER at the re
cells. strictive temperature [262]. In addition to BiP/Kar2p
In an in vitra system containing wheat germ extract at least one other luminal HSP70-Iike protein has also
and maize microsomes, cytosolic HSP70 has been been shown to be involved in protein translocation into
shown to greatly stimulate import of a model protein the yeast and mammalian ER [42,52].
into the ER [173]. This finding suggest that, when Kar2p has been proposed to be required to reel
components of the targeting machinery are limiting, in the nascent chain, binding to it soon after it has
HSP70 may be important in maintaining the precur appeared at the 1uminal side of the translocon and im
sor protein in a translocation-competent state before it posing a vectoriality to the transfer of the polypeptide.
is targeted to the ER membrane. Consistent with this This function (like the one of cytosolic chaperones)
view, HSP70 is found in association with polypeptides might be much more crucial for post-translationally
synthesized in vitra in the absence of ER derived mi translocated proteins, since protein synthesis itself can
crosomes [173]. HSP70-like proteins have also been push cotranslationally translocated chains through the
shown to be involved in the process of protein im translocation pore. Indeed BiP does not see m to be
port into the yeast ER [31, 50]. Co-translationally and strictly required for cotranslational translocation in
post-translationally translocated proteins were shown mammals [77, 276].
to differ in their degree of dependence on cytoso In addition to the components involved in target
lic HSP70-like proteins, suggesting that the action of ing and translocation, the translocon contains aseries
4
of polypeptides which catalyze the introduction of quences that also function as transmembrane domains.
covalent modifications into the nascent chain. The N In this case proteins with either a Ncyt/Clum(Type II)
terminal signal of secretory proteins (and of type I or a Nlum/Ccyt (Type III) orientation can be gener
membrane proteins, see below) is co-translationally ated, depending on whether the signal anchor se
removed and this step is required to release the pro quence promotes the translocation of the C-terminal or
tein in the ER lumen. The enzyme responsible for this N-terminal part of the polypeptide. Similarly, multi
processing (signal peptidase) [56] is located at the site spanning proteins can be classified as type I, II or
of the translocon and efficiently clips the nascent chain IlI, depending on the kind of transmembrane domain
as soon as it is long enough to expose the cleavage which is first targeted to the ER. In type II and type
site on the luminal face of the ER membrane. Mu III proteins, various factors determine whether the
tations that alter crucial amino acids preceding the N -terminal or C-terminal part of the pro tein is translo
cleavage site can block processing and convert a sol cated [265], but the distribution of charged residues
uble protein into a membrane-spanning one, which flanking the transmembrane segment appears to be the
remains anchored to the ER membrane through its most impotant [84, 265].
signal sequence. This is best exemplified by the case Targeting of these kinds of membrane proteins to
of one zein mutant polypeptide. Zeins are the main the ER is also mediated by SRP and Sec6 I [103, 107,
storage proteins in maize kerneis (see detailed de 115] and the signals and mechanisms that mediate
scription later) and are translocated into the ER in a their insertion into the ER have been conserved during
SRP-dependent fashion [26]. In the fioury 2 mutant evolution. This is shown by the successful expression
of maize, an Ala-to-Val substitution in the position of functional plant aquaporins (which are membrane
immediately preceding the cleavage site blocks the re proteins with a complex topology) in Xenopus laevis
moval of the signal peptide from a zein polypeptide oocytes [167, 168]. Arabidopsis thaliana 3-hydroxy-
[35] which is consequently tethered to the ER mem 3-methylglutaryl coenzyme A reductase is an example
brane [74]. Similarly, an uncleaved zein signal peptide of an ER membrane protein whose insertion into mem
was able to function as a transmembrane domain when branes is SRP-mediated [25]. The enzyme contains
fused to maize alcohol dehydrogenase [228]. two transmembrane domains and both of them can
Another common covalent modification of the interact with SRP. However, it should be noted that
nascent chains is the addition of N -Iinked oligosaccha in certain multi-spanning membrane proteins, SRP in
ride side-chains [136]. Although the machinery that volvement is required only for the insertion of the first
catalyzes N-Iinked glycosylation may be considered transmembrane segment [178,270].
part of the translocon, the details of the reaction and The insertion of one further class of membrane
its role in the biosynthesis of proteins in the plant proteins is instead mediated by a different, still iII
endomembrane system will be described in another defined SRP/Sec61-independent mechanism. In these
section of this review. proteins, ER targeting and insertion oceur post
translationally and are due to the presence of hy
Membrane proteins are inserted into the ER drophobie stretehes located at the N or C terminus
of the protein [18]. This class of proteins includes
membrane via either SRP-dependent or
cytochrome bs, a highly eonserved integral ER mem
SRP-independent mechanisms
brane protein which is known to be involved in lipid
On the basis of their topology relative to the lipid synthesis in plants. The protein can be inserted post
bilayer and the kind of sequence that directs their translationally into isolated microsomal membranes
insertion, membrane proteins with a single membrane and eontains aC-terminal hydrophobic sequenee
spanning domain can be divided into different classes whieh is essential for ER targeting [240]. Insertion
[244]. Type I proteins contain a cleaved signal peptide requires a protease-sensitive component present on the
and are anchored to the membrane by a 'stop transfer' surfaee of the microsomes, suggesting that interaction
sequence, which blocks the translocation of the C with a still unidentified reeeptor mediates the speeific
terminal part of the protein. This results in a N/umlCcyt targeting of cytochrome bs into the ER membrane.
orientation. Other membrane proteins do not contain Another protein, whieh has been proposed to be
a cleavable signal at their N-terminus and their inser targeted to the ER membrane by this alternative path
tion into the ER membranes is mediated by 'signal way, is the 6 kDa protein of tobacco etch potyvirus
anchor' sequences, i.e. internal uncleaved signal se- (TEV). In TEV-infeeted eells, the 6 kDa protein di-