Synthesis, processing and export of cytoplasmic endo-beta-1,4-xylanase from barley aleurone during germination

We have identified the major endo-beta-1,4-xylanase (XYN-1) in the aleurone of germinating barley grain, and show that it is expressed as a precursor of Mr 61 500 with both N- and C-terminal propeptides. XYN-1 is synthesized as an inactive enzyme in the cytoplasm, and only becomes active at a late stage of germination when the aleurone ceases to secrete hydrolases. A series of processing steps, mediated in part by aleurone cysteine endoproteases, yields a mature active enzyme of Mr 34 000. Processing and extracellular release of the mature enzyme coincide with the programmed cell death (PCD)-regulated disintegration of aleurone cells. We discuss the significance of delayed aleurone cell-wall degradation by endoxylanases in relation to the secretory capacity of the aleurone, and propose a novel role for aleurone PCD in facilitating the export of hydrolases.

14-3-3 proteins interact with a 13-lipoxygenase, but not with a 9-lipoxygenase

Associations between lipoxygenases (Lox) and 14-3-3 proteins were demonstrated by two different methods. First, immunoprecipitation experiments, using isoenzyme-specific monoclonal Lox antibodies, showed that 14-3-3 proteins co-precipitate with 13-Lox, but not with the 9-Lox from barley. Second, interactions between 13-Lox and 14-3-3 were established by surface plasmon resonance studies, showing that 13-Lox binds with 14-3-3 proteins in a concentration-dependent manner. The interactions between 14-3-3 proteins and 13-Lox may reveal their role during plant development.

Subcellular differences in post-translational modification of barley 14-3-3 proteins

Expression and post-translational modification of barley 14-3-3 isoforms, 14-3-3A, 14-3-3B and 14-3-3C, were investigated using isoform-specific antibodies. Although all three isoforms were shown to be present in the cytosolic, the nuclear and the microsomal cell fractions, differences in post-translational modification were identified for the different cell fractions. Germination-related modifications of 14-3-3 proteins were observed in the cytosol and the microsomal fraction, but not in the nucleus. In vitro proteolytic cleavage of 14-3-3 proteins using trypsin suggests that for 14-3-3A this change was caused by proteolytic cleavage of the unconserved C-terminal region.

The tobacco homolog of mammalian calreticulin is present in protein complexes in vivo

The analysis of protein sorting signals responsible for the retention of reticuloplasmins (RPLs), a group of soluble proteins that reside in the lumen of the endoplasmic reticulum (ER), has revealed a structural similarity between mammalian and plant ER retention signals. We present evidence that the corresponding epitope is conserved in a vast family of soluble ER resident proteins. Microsequences of RPL60 and RPL90, two abundant members of this family, show high sequence similarity with mammalian calreticulin and endoplasmin. RPL60/calreticulin cofractionates and costains with the lumenal binding protein (BiP). Both proteins were detected in the nuclear envelope and the ER, and in mitotic cells in association with the spindle apparatus and the phragmoplast. Immunoprecipitation of proteins from in vivo-labeled cells demonstrated that RPL60/calreticulin is associated with other polypeptides in a stress- and ATP-dependent fashion. RPL60/calreticulin transcript levels increased rapidly in abundance during the proliferation of the secretory apparatus and the onset of hydrolase secretion in gibberellic acid-treated barley aleurone cells. This induction profile is identical to that of the well-characterized ER chaperones BiP and endoplasmin. However, expression patterns in response to different stress conditions as well as tissue-specific expression patterns indicate that these genes are differentially regulated and may not act in concert.

Microtubule perturbation inhibits intracellular transport of an apical membrane glycoprotein in a substrate-dependent manner in polarized Madin-Darby canine kidney epithelial cells

The effects of microtubule perturbation on the transport of two different viral glycoproteins were examined in infected Madin-Darby canine kidney (MDCK) cells grown on both permeable and solid substrata. Quantitative biochemical analysis showed that the microtubule-depolymerizing drug nocodazole inhibited arrival of influenza hemagglutinin on the apical plasma membrane in MDCK cells grown on both substrata. In contrast, the microtubule-stabilizing drug taxol inhibited apical appearance of hemagglutinin only when MDCK cells were grown on permeable substrata. On the basis of hemagglutinin mobility on sodium dodecyl sulfate gels and its sensitivity to endo H, it was evident that nocodazole and taxol arrested hemagglutinin at different intracellular sites. Neither drug caused a significant increase in the amount of hemagglutinin detected on the basolateral plasma membrane domain. In addition, neither drug had any noticeable effect on the transport of the vesicular stomatitis virus (VSV)-G protein to the basolateral surface. These results shed light on previous conflicting reports using this model system and support the hypothesis that microtubules play a role in the delivery of membrane glycoproteins to the apical, but not the basolateral, domain of epithelial cells.