Functional complementation of a yeast vesicular transport mutation ypt1-1 by a Brassica napus cDNA clone encoding a small GTP-binding protein

RAB GTPases and SNAREs at the trans-Golgi network in plants

Membrane traffic is a fundamental cellular system to exchange proteins and membrane lipids among single membrane-bound organelles or between an organelle and the plasma membrane in order to keep integrity of the endomembrane system. RAB GTPases and SNARE proteins, the key regulators of membrane traffic, are conserved broadly among eukaryotic species. However, genome-wide analyses showed that organization of RABs and SNAREs that regulate the post-Golgi transport pathways is greatly diversified in plants compared to other model eukaryotes. Furthermore, some organelles acquired unique properties in plant lineages. Like in other eukaryotic systems, the trans-Golgi network of plants coordinates secretion and vacuolar transport; however, uniquely in plants, it also acts as a platform for endocytic transport and recycling. In this review, we focus on RAB GTPases and SNAREs that function at the TGN, and summarize how these regulators perform to control different transport pathways at the plant TGN. We also highlight the current knowledge of RABs and SNAREs' role in regulation of plant development and plant responses to environmental stimuli.

Plant RABs: Role in Development and in Abiotic and Biotic Stress Responses

Endosomal trafficking plays an integral role in various eukaryotic cellular activities and is vital for higher-order functions in multicellular organisms. RAB GTPases are important proteins that influence various aspects of membrane traffic, which consequently influence many cellular functions and responses. Compared to yeast and mammals, plants have evolved a unique set of plant-specific RABs that play a significant role in their development. RABs form the largest family of small guanosine triphosphate (GTP)-binding proteins, and are divided into eight sub-families named RAB1, RAB2, RAB5, RAB6, RAB7, RAB8, RAB11 and RAB18. Recent studies on different species suggest that RAB proteins play crucial roles in intracellular trafficking and cytokinesis, in autophagy, plant microbe interactions and in biotic and abiotic stress responses. This review recaptures and summarizes the roles of RABs in plant cell functions and in enhancing plant survival under stress conditions.

Formation and maintenance of the Golgi apparatus in plant cells

The Golgi apparatus plays essential roles in intracellular trafficking, protein and lipid modification, and polysaccharide synthesis in eukaryotic cells. It is well known for its unique stacked structure, which is conserved among most eukaryotes. However, the mechanisms of biogenesis and maintenance of the structure, which are deeply related to ER-Golgi and intra-Golgi transport systems, have long been mysterious. Now having extremely powerful microscopic technologies developed for live-cell imaging, the plant Golgi apparatus provides an ideal system to resolve the question. The plant Golgi apparatus has unique features that are not conserved in other kingdoms, which will also give new insights into the Golgi functions in plant life. In this review, we will summarize the features of the plant Golgi apparatus and transport mechanisms around it, with a focus on recent advances in Golgi biogenesis by live imaging of plants cells.

Brassica napus Rop GTPases and their expression in microspore cultures

Androgenesis in plants involves a shift in development that causes cultured microspore cells to form embryos rather than continue to develop pollen. In Brassica napus microspore culture a mild heat stress is used to switch on embryo development. An early hallmark of embryogenesis in this system is a symmetrical division of the nucleus instead of the asymmetric division that occurs during pollen formation. ROP GTPases act as molecular switches in a variety of developmental processes; therefore, the current study was initiated to examine whether they might be involved in androgenesis. Five distinct Rop genes with nucleic acid similarities ranging from 82 to 93% to Arabidopsis Rop1 were isolated from B. napus cv Topas. A Southern blot hybridization with a BnRop sequence probe suggested that there are 11-15 ROP gene family members in B. napus. RT-PCR reactions with PCR primers specific to BnRop5, BnRop6, BnRop9 and BnRop10 showed that expression of the BnRop5 was restricted to pollen but the others were detected in leaf, root, stem and pollen tissue. Pollen-like cells obtained from 3-day-old cultures by flow cytometric sorting had BnRop5 transcript levels that were 2.8 times higher than in flow sorted embryogenic microspores. Conversely, the BnRop9 transcript levels were 2.5-fold higher in the embryogenic cells than in the pollen-like cells. The potential involvement of specific ROPs in early stage microspore culture responses is discussed.

Arf1 GTPase plays roles in the protein traffic between the endoplasmic reticulum and the Golgi apparatus in tobacco and Arabidopsis cultured cells

Arf GTPases are known to be key regulators of vesicle budding in various steps of membrane traffic in yeast and animal cells. We cloned the Arabidopsis Arf1 homologue, AtArf1, and examined its function. AtArf1 complements yeast arf1 arf2 mutants and its GFP-fusion is localized to the Golgi apparatus in plant cells like its animal counterpart. The expression of dominant negative mutants of AtArf1 in tobacco and Arabidopsis cultured cells affected the localization of co-expressed GFP-tagged proteins in a variety of ways. AtArf1 Q71L and AtArf1 T31N, GTP- and GDP-fixed mutants, respectively, changed the localization of a cis-Golgi marker, AtErd2-GFP, from the Golgi apparatus to the endoplasmic reticulum but not that of GFP-AtRer1B or GFP-AtSed5. GFP-AtRer1B and GFP-AtSed5 were accumulated in aberrant structures of the Golgi by AtArf1 Q71L. A soluble vacuolar protein, sporamin-GFP, was also located to the ER by AtArf1 Q71L. These results indicate that AtArf1 play roles in the vesicular transport between the ER and the Golgi and in the maintenance of the normal Golgi organization in plant cells.

Rab2 GTPase regulates vesicle trafficking between the endoplasmic reticulum and the Golgi bodies and is important to pollen tube growth

Pollen tube elongation depends on the secretion of large amounts of membrane and cell wall materials at the pollen tube tip to sustain rapid growth. A large family of RAS-related small GTPases, Rabs or Ypts, is known to regulate both anterograde and retrograde trafficking of transport vesicles between different endomembrane compartments and the plasma membrane in mammalian and yeast cells. Studies on the functional roles of analogous plant proteins are emerging. We report here that a tobacco pollen-predominant Rab2, NtRab2, functions in the secretory pathway between the endoplasmic reticulum and the Golgi in elongating pollen tubes. Green fluorescent protein-NtRab2 fusion protein localized to the Golgi bodies in elongating pollen tubes. Dominant-negative mutations in NtRab2 proteins inhibited their Golgi localization, blocked the delivery of Golgi-resident as well as plasmalemma and secreted proteins to their normal locations, and inhibited pollen tube growth. On the other hand, when green fluorescent protein-NtRab2 was over-expressed in transiently transformed leaf protoplasts and epidermal cells, in which NtRab2 mRNA have not been observed to accumulate to detectable levels, these proteins did not target efficiently to Golgi bodies. Together, these observations indicate that NtRab2 is important for trafficking between the endoplasmic reticulum and the Golgi bodies in pollen tubes and may be specialized to optimally support the high secretory demands in these tip growth cells.

Broad bean wilt virus Causes Necrotic Symptoms and Generates Defective RNAs in Capsicum annuum

ABSTRACT A virus was isolated from hot pepper (Capsicum annuum cv. Hyang Chon) growing in Korea and displaying necrotic spots or streaks on leaves and stems followed by stunting and death of plants. Morphological and host range analyses of extracts from infected plants suggested that the causal agent of disease was a Broad bean wilt virus (BBWV), and the virus was tentatively named a Korean isolate of BBWV (BBWV-K). When the isolate was back-inoculated onto hot pepper plants, it induced symptoms similar to those of naturally infected hot pepper in the field. Two coat proteins (CPs) of 44 and 22 kDa, corresponding to a large CP and a small CP, respectively, were identified from the virus, and both reacted specifically with polyclonal antibody to BBWV 2. The complete nucleotide sequences of RNA 1 and RNA 2 of the isolate were determined from cDNA clones. The deduced amino acid sequence data from the putative proteins encoded by RNA 1 and 2 of the BBWV-K indicated a closer relationship with the isolates of BBWV 2 than BBWV 1. However, sequence comparison of the 5' noncoding regions of the viruses differentiates BBWV-K from other BBWV 2 isolates. Another distinctive feature of the BBWV-K is that it generates defective RNAs in hot pepper exhibiting necrotic symptoms, which is the first report of defective RNAs in the Fabavirus genera of BBWVs.

A dominant negative mutant of sar1 GTPase inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus in tobacco and Arabidopsis cultured cells

Protein secretion plays an important role in plant cells as it does in animal and yeast cells, but the tools to study molecular events of plant secretion are very limited. We have focused on the Sar1 GTPase, which is essential for the vesicle formation from the endoplasmic reticulum (ER) in yeast, and have previously shown that tobacco and Arabidopsis SAR1 complement yeast sar1 mutants. In this study, we have established a transient expression system of GFP-fusion proteins in tobacco and Arabidopsis cultured cells. By utilizing confocal laser scanning microscopy, we demonstrate that a dominant negative mutant of Arabidopsis Sar1 inhibits the ER-to-Golgi transport of Golgi membrane proteins, AtErd2 and AtRer1B, and locates them to the ER. The same mutant Sar1 also blocks the exit from the ER of a vacuolar storage protein, sporamin. These results not only provide the first evidence that the Sar1 GTPase functions in the ER-to-Golgi transport in plant cells, but also prove that conditional expression of dominant mutants of secretory machinery can be a useful tool in manipulating vesicular trafficking.

Overexpression of PRA2, a Rab/Ypt-family small GTPase from Pea Pisum sativum, aggravates the growth defect of yeast ypt mutants

A large number of Rab/Ypt-family small GTPases have been identified from higher plants. While some of them can complement yeast ypt mutants, the expression of Arabidopsis Ara4 protein aggravated the growth defect of a subset of ypt mutants, probably because of the titration of common regulator(s) of yeast Ypt proteins [Ueda, T. et al. (1996) Plant Cell, 8: 2079-20911. PRA2 from pea Pisum sativum encodes an interesting Rab GTPase whose expression is regulated by light [Yoshida, K. et al. (1993) Proc. Natl. Acad. Sci. USA, 90: 6636-6640]. We examined whether PRA2 complements any of the yeast ypt mutants and found again that PRA2 does not complement but rather confers the growth defect to some of the ypt mutants. No growth defect was observed when PRA2 was expressed in the wild-type yeast cells. Unlike the case of Ara4, neither Arabidopsis nor yeast GDI remedied the growth defect by Pra2, indicating that the mechanism of the exacerbation is different. Mutational analysis of PRA2 suggests that the growth inhibition can be ascribed to unidentified factor(s) which prefers the GTP-bound form of Pra2. This yeast system will be useful for identifying such putative regulatory factor(s) from yeast and plants and analyzing their interactions with Pra2.

Guanine-nucleotide binding and hydrolyzing kinetics of ORrab2, a rice small GTP-binding protein expressed in Escherichia coli

The ORrab2 gene encodes a GTP-binding protein of 23.169 kDa. The deduced amino acid sequence shows that ORrab2 has the motifs conserved among small GTP-binding proteins in plants and that it shares sequence identity with Atrab2 (93.0%), Hrab2 (85.2%), Hrab4 (51.9%), Hrab1 (46.2%), YPT (40.7%), Hrab3B (40.0%), Hrab3A (38.1%), SEC4 (38.1%), Hrab5 (34.3%) and Hrab6 (32.4%). To analyze the biochemical properties of this protein, an ORrab2 cDNA was overexpressed in Escherichia coli and the protein purified by Ni2+-nitrilotriacetic acid agarose and hydroxyapatite column chromatography. The molecular mass of the protein bearing a His-tag is approximately 28.2 kDa. The guanine-nucleotide binding and hydrolyzing activity of ORrab2 increased with non-ionic C12E10 (polyoxyethylene 10-lauryl ether) and ionic Chaps detergent treatment. ORrab2 bound maximally 1.03 mol of [gamma-35S]GTP[S]/mol of protein with a Kd value of 56.83 nM. The ratios k(off GDP)/k(off GTP) of ORrab2 were 3.63 for the control, 3.7 in the presence of C12E10, and 3.83 with Chaps, indicating that ORrab2 has a higher affinity for GTP than GDP. The rate (k(cat)) of Pi release against [gamma-32P]GTP bound ORrab2 in a steady state and the rate of hydrolysis of [gamma-32P]GTP (kGTPase) were calculated to be 432 x 10(-4) +/- 8 x 10(-4) min(-1) and 172 x 10(-4) +/- 2 x 10(-4) min(-1), respectively, in the presence of 0.1% C12E10 and 1 mM MgSO4.

Insulin-induced maturation of Xenopus oocytes is inhibited by microinjection of a Brassica napus cDNA clone with high similarity to a mammalian receptor for activated protein kinase C

A cDNA clone encoding a WD-40 repeat protein (BGB1) was characterized in Brassica napus L. The clone contained an open reading frame of 327 amino acid residues almost entirely composed of seven segments of WD-40 repeats. Among the WD-40 repeat proteins, BGB1 showed high similarity (63% identity) to a rat intracellular receptor for protein kinase C (RACK1) that functions in the translocation of activated protein kinase C (PKC) from the cytosolic fraction to the membrane fraction. BGB1 also had two sequence motifs involved in binding of RACK1 to PKC. The cDNA clone, when carried in a Xenopus oocyte expression vector and injected into Xenopus laevis oocytes, inhibited insulin-induced maturation of the oocytes, a PKC-mediated pathway, and this inhibition was accompanied by reduction of PKC in the membrane fraction, as in the case of mammalian RACKs. The data show that BGB1 shares some common functional characteristics with the mammalian RACK1 along with the structural similarity, suggesting that a mammalian RACK1-related cellular process might be operating in plants. Southern blot analyses of the genome of B. napus and Arabidopsis thaliana (L.) Heynh. revealed that BGB1-related genes constitute a small multigene family in both species. An approximately 1.4-kb transcript was constitutively expressed in all organs examined.

Transport of proteins in eukaryotic cells: more questions ahead

Some newly synthesized proteins contain signals that direct their transport to their final location within or outside of the cell. Targeting signals are recognized by specific protein receptors located either in the cytoplasm or in the membrane of the target organelle. Specific membrane protein complexes are involved in insertion and translocation of polypeptides across the membranes. Often, additional targeting signals are required for a polypeptide to be further transported to its site of function. In this review, we will describe the trafficking of proteins to various cellular organelles (nucleus, chloroplasts, mitochondria, peroxisomes) with emphasis on transport to and through the secretory pathway.