1021-4437/02/4902- $27.00 © 2002
Russian Journal of Plant Physiology, Vol. 49, No. 2, 2002, pp. 255–268. Translated from Fiziologiya Rastenii, Vol. 49, No. 2, 2002, pp. 286–301.
Original Russian Text Copyright © 2002 by Sharova.
In plant cells, there are more than 30 compartments
differing in their structure and metabolic processes
because of the difference in their protein composition.
The differentiation of organelles and the maintenance
and adaptive changes of the cell ultrastructure depend
on the correct targeting of proteins synthesized mainly
in the cytosol. Diverse methods have been applied in
the studies of the intracellular protein transport .
Among them are pulse labeling of the molecules com-
bined with the ultrastructural analysis and cell-content
assays of protein transport into
various organelles and of the donor and acceptor mem-
brane fusion, and the methods of chemical cross-link-
ing of proteins interacting in the process of transport.
During the last decade, considerable progress has been
made due to genetic approaches, i.e., using mutants
exhibiting impaired protein targeting and gene con-
structs permitting identiﬁcation of the peptide domains
that carry information concerning the protein subcellu-
lar localization. As a result, the principal cellular pro-
tein-transporting systems were elucidated. Their com-
parison in diverse organisms revealed the evolution-
arily conserved mechanisms for protein entrance into
the ER, nucleus, mitochondria, and microbodies. In
plant cells, the pathways of protein transport into plas-
tids and various vacuole types were also characterized.
All proteins synthesized in the cytosol can be divided
into the two groups: (1) the proteins synthesized on free
polysomes and transported into the nucleus, chloro-
plasts, mitochondria, and microbodies, and (2) the pro-
teins synthesized on the polysomes attached to the ER
and directed along the secretory pathway.
PROTEIN TRANSPORT ALONG
THE SECRETORY PATHWAY
Plant cells continuously secrete enzymes as well as
structural and regulatory proteins of the cell walls. The
intracellular transport of these proteins occurs along the
secretory pathway, which includes: (1) the cotransla-
tional crossing of the ER membrane; (2) the removal of
the signal peptide, N-glycosylation and folding of pro-
teins in the ER lumen; (3) protein transport in Golgi cis-
ternae including the conversion of the N-glycan chain
and protein O-glycosylation; (4) protein loading into
the vesicles transporting them to the plasma membrane
and vacuoles; and (5) the fusion of transport vesicles
with the acceptor membranes.
Protein Transport into the Lumen
of the Endoplasmic Reticulum
The process of cotranslational protein translocation
into the ER lumen has been described for mammals and
Protein Transport in Plant Cells
E. I. Sharova
Institute of Biology, St. Petersburg State University, Oranienbaumskoe sh. 2, Staryi Peterhof, St. Petersburg, 198504 Russia;
fax: 7 (812) 427-7310; e-mail: email@example.com
Received November 13, 2000
—The subcellular localization and secretion of proteins synthesized in the cytosol are determined by
short amino acid sequences in their molecules. N-terminal transit peptides provide for protein translocation
across the membranes of the ER, mitochondria, plastids, and microbodies. Later, these peptides are cleaved off
by processing peptidases. C-terminal peptides direct some proteins into microbodies and vacuoles. Transport
into the nucleus and insertion in the membranes are determined by the speciﬁc sequences that reside in the mol-
ecule of the mature protein. Speciﬁc receptors associated with the protein-translocating channel recognize tran-
sit peptides. Protein unfolding is required for successful protein transport through these channels. Chaperones
maintain proteins in such a state. Folded proteins cross the nuclear pore complex and the membrane of micro-
bodies. Protein transport is tightly associated with their processing. During the vesicular protein transport
within the endomembrane system (ER, Golgi apparatus, plasma membrane, and vacuoles), correct protein tar-
geting is ensured by protein sorting during vesicle loading, the assembly of corresponding protein coats, vesicle
transport to the acceptor membrane, and speciﬁc membrane fusion.
Key words: protein transport - transit polypeptide - vesicles - secretion - chaperones - endoplasmic reticulum -
Golgi apparatus - vacuoles - microbodies - nucleus - plastids - mitochondria
: BiP—binding protein; CAMV—cauliﬂower
mosaic virus; COP—protein coat of the transport vesicles; ER—
endoplasmic reticulum; Hsp—heat-shock protein(s); NLS—
nucleus localization signal; PTS—peroxisome targeting signal;
SRP—signal recognition particle; TGN—
TIC—translocon at the inner membrane of chloroplast envelope;
TIM—translocase of the inner mitochondrial membrane; TOC—
translocon at the outer membrane of chloroplast envelope;
TOM—translocase at the outer mitochondrial membrane.