ABSTRACT
14-3-3 proteins regulate a wide range of target proteins via direct protein-protein interactions. The target-binding domain in 14-3-3 proteins is highly conserved, suggesting similar biochemical properties for all 14-3-3s. However, higher eukaryotes possess multiple 14-3-3 genes, and these genes exhibit diverse patterns of gene expression within any one organism. This tends to suggest specific functions for particular genes. Some biochemical data suggest 14-3-3 isoform-specific protein-protein interactions, whereas other studies conclude that apparent isoform-specificity is the result of differences in expression patterns rather than in the biochemical properties of 14-3-3 isoforms. Here we discuss evidence that demonstrates that the expression levels of 14-3-3 proteins in cells are important for regulating the activity of their target proteins, and further that the elimination of individual 14-3-3 isoforms can result in detectable phenotypes. We also examine evidence that 14-3-3 isoform specificity can in some cases reflect differing biochemical properties as well as differential transcriptional regulation.
Subject(s)
MAP Kinase Signaling System/physiology , Plants/enzymology , Tyrosine 3-Monooxygenase/metabolism , 14-3-3 Proteins , Gene Expression Regulation, Plant , Isoenzymes/metabolism , Plants/genetics , Tyrosine 3-Monooxygenase/geneticsABSTRACT
Soybean peribacteroid membrane (PBM) proteins were isolated from nitrogen-fixing root nodules and subjected to N-terminal sequencing. Sequence data from 17 putative PBM proteins were obtained. Six of these proteins are homologous to proteins of known function. These include three chaperones (HSP60, BiP [HSP70], and PDI) and two proteases (a serine and a thiol protease), all of which are involved in some aspect of protein processing in plants. The PBM homologs of these proteins may play roles in protein translocation, folding, maturation, or degradation in symbiosomes. Two proteins are homologous to known, nodule-specific proteins from soybean, nodulin 53b and nodulin 26B. Although the function of these nodulins is unknown, nodulin 53b has independently been shown to be associated with the PBM. All of the eight proteins with identifiable homologs are likely to be peripheral rather than integral membrane proteins. Possible reasons for this apparent bias are discussed. The identification of homologs of HSP70 and HSP60 associated with the PBM is the first evidence that the molecular machinery for co- or post-translational import of cytoplasmic proteins is present in symbiosomes. This has important implications for the biogenesis of this unique, nitrogen-fixing organelle.
Subject(s)
Glycine max/physiology , Membrane Proteins/chemistry , Plant Proteins/chemistry , Plant Roots/physiology , Rhizobium/physiology , Symbiosis/physiology , Amino Acid Sequence , Databases, Factual , Electrophoresis, Gel, Two-Dimensional , Electrophoresis, Polyacrylamide Gel , Membrane Proteins/isolation & purification , Molecular Sequence Data , Plant Proteins/isolation & purification , Plant Roots/chemistry , Rhizobium/chemistry , Sequence Analysis, ProteinABSTRACT
A full-length cDNA, GmNRT2, encoding a putative high-affinity nitrate transporter was isolated from a Glycine max (L.) root cDNA library and sequenced. The deduced GmNRT2 protein is 530 amino acids in length and contains 12 putative membrane-spanning domains and a long, hydrophilic C-terminal domain. GmNRT2 is related to high-affinity nitrate transporters in the eukaryotes Chlamydomonas reinhardtii and Aspergillus nidulans, and to putative high-affinity nitrate transporters in barley and tobacco. Southern blot analysis indicated that GmNRT2 is part of a small, multigene family in soybean. Expression of GmNRT2 in roots was regulated by the type of nitrogen source provided to plants: GmNRT2 mRNA levels were barely detectable in ammonium-grown plants, higher in nitrogen-deprived plants, and highest in nitrate-grown plants. Induction of GmNRT2 mRNA levels in roots occurred within 1 h after exposure of plants to nitrate. Nitrate induction of GmNRT2 mRNA levels was accompanied by a fourfold increase in net nitrate uptake by soybean roots at 100 microM external nitrate. The molecular and physiological evidence indicates that GmNRT2 is probably a high-affinity nitrate transporter involved in nitrate uptake by soybean roots.
Subject(s)
Anion Transport Proteins , Carrier Proteins/genetics , Gene Expression Regulation, Plant , Glycine max/genetics , Nitrates/metabolism , Plant Proteins/genetics , Soybean Proteins , Amino Acid Sequence , Base Sequence , Biological Transport , Carrier Proteins/metabolism , DNA, Complementary , DNA, Plant , Molecular Sequence Data , Multigene Family , Nitrate Transporters , Plant Proteins/metabolism , Plant Roots/metabolism , RNA, Messenger , Sequence Homology, Amino AcidABSTRACT
Exogenous abscisic acid (ABA) induced the alcohol dehydrogenase gene (Adh) in Arabidopsis roots. Both the G-box-1 element and the GT/GC motifs (anaerobic response element) were required for Adh inducibility. Measurement of endogenous ABA levels during stress treatment showed that ABA levels increased during dehydration treatment but not following exposure to either hypoxia or low temperature. Arabidopsis ABA mutants (aba1 and abi2) displayed reduced Adh mRNA induction levels following either dehydration treatment or exogenous application of ABA. Low-oxygen response was slightly increased in the aba1 mutant but was unchanged in abi2. Low-temperature response was unaffected in both aba1 and abi2 mutants. Our results indicate that, although induction of the Adh gene by ABA, dehydration, and low temperature required the same cis-acting promoter elements, their regulatory pathways were at least partially separated in a combined dehydration/ABA pathway and an ABA-independent low-temperature pathway. These pathways were in turn independent of a third signal transduction pathway leading to low-oxygen response, which did not involve either ABA or the G-box-1 promoter element.
