Plant Golgi-associated vesicles contain a novel alpha-actinin-like protein

Anti-tropomyosin antibodies co-localise with actin microfilaments and label plasmodesmata

The actin cytoskeleton and associated actin-binding proteins form a complex network involved in a number of fundamental cellular processes including intracellular trafficking. In plants, both actin and myosin have been localised to plasmodesmata, and thus it is likely that other actin-binding proteins are also associated with plasmodesmata structure or function. A 75-kDa protein, enriched in plasmodesmata-rich cell wall extracts from the green alga Chara corallina, was sequenced and found to contain three peptides with similarity to the animal actin-binding protein tropomyosin. Western blot analysis with anti-tropomyosin antibodies confirmed the identity of this 75-kDa protein as a tropomyosin-like protein and further identified an additional 55-kDa protein, while immunofluorescence microscopy localised the antibodies to plasmodesmata and to the subcortical actin bundles and associated structures. The anti-tropomyosin antibodies detected a single protein at 42.5 kDa in Arabidopsis thaliana extracts and two proteins at 58.5 and 54 kDa in leek extracts, and these localised to plasmodesmata and the cell plate in A. thaliana and to plasmodesmata in leek tissue. Tropomyosin is an actin-binding protein thought to be involved in a range of functions associated with the actin cytoskeleton, including the regulation of myosin binding to actin filaments, but to date no tropomyosin-like proteins have been conclusively identified in plant genomes. Our data suggests that a tropomyosin-like protein is associated with plasmodesmata.

Distribution of an ankyrin-repeat protein on the endoplasmic reticulum in Arabidopsis

There are many ankyrin-repeat proteins in plant cells. However, the distribution and function of these proteins are mostly unclear. By reverse transcription-polymerase chain reaction, a gene encoding an ankyrin-like protein was cloned from Arabidopsis and named AtANK1 (GenBank accession no. NM_120340). The 6-His-tagged AtAnk1-N fusion protein was affinity-purified and its rabbit polyclonal antibody was obtained. Immuno-blotting with the purified anti-AtAnk1-N polyclonal antibody revealed that the relative molecular weight of the AtANK1 protein was about 76 kDa. By immunofluorescence labeling and immuno-gold labeling with the purified anti-AtAnk1-N polyclonal antibody, coupled with confocal and transmission electron microscopy observation, AtANK1 was found to be distributed on the membrane of the endoplasmic reticulum in Arabidopsis cells. Based on these results, we suggested that AtANK1 might be involved in endoplasmic reticulum-related protein localization and sorting in plant cells.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

Modifications of Golgi complex in chondrocytes from osteoarthrotic (OA) rat cartilage

The status of the Golgi complex in normal vs osteoarthrotic (OA) cartilage has not yet been studied. A monoclonal antibody, MAb 58-K-9, allowed scoring of Golgi labeling intensity. In addition, ultrastructural assessment enabled us to focus on the distribution and relation between the endoplasmic reticulum (ER) and Golgi membranes. The study was performed in both normal and partially menisectomized OA-induced rat cartilage 20 and 45 days after surgery. Comparing Golgi immunolabeling intensities (mean +/- SEM) revealed a highly significant difference between normal (9.98 +/- 1.25), 20-day (2.49 +/- 0.34), and 45-day (0.82 +/- 0.22) cartilage. Moreover, chondrocytes from normal cartilage displayed 71.18% of labeling intensity in contrast to OA cartilage, in which chondrocyte labeling intensities were 24.95% (20 days) and 8.11% (45 days). OA chondrocytes appeared to display an overall reduction in Golgi labeling intensity, suggesting disruption of this organelle as the OA damage progressed. Interestingly, many 20-day OA-induced chondrocytes exhibited bubble-like Golgi immunolabeling compartmentalizing the cytoplasm, concomitant with putative apoptotic nuclear changes. At the same time, OA chondrocytes with a typical ultrastructural apoptotic pattern revealed a prominent ER gathered together with Golgi vesicles and saccules, also appearing to compartmentalize chondrocyte cytoplasm. We speculate about the role of Golgi modifications and apoptosis in OA pathogenesis.

F-actin-dependent endocytosis of cell wall pectins in meristematic root cells. Insights from brefeldin A-induced compartments

Brefeldin A (BFA) inhibits exocytosis but allows endocytosis, making it a valuable agent to identify molecules that recycle at cell peripheries. In plants, formation of large intracellular compartments in response to BFA treatment is a unique feature of some, but not all, cells. Here, we have analyzed assembly and distribution of BFA compartments in development- and tissue-specific contexts of growing maize (Zea mays) root apices. Surprisingly, these unique compartments formed only in meristematic cells of the root body. On the other hand, BFA compartments were absent from secretory cells of root cap periphery, metaxylem cells, and most elongating cells, all of which are active in exocytosis. We report that cell wall pectin epitopes counting rhamnogalacturonan II dimers cross-linked by borate diol diester, partially esterified (up to 40%) homogalacturonan pectins, and (1-->4)-beta-D-galactan side chains of rhamnogalacturonan I were internalized into BFA compartments. In contrast, Golgi-derived secretory (esterified up to 80%) homogalacturonan pectins localized to the cytoplasm in control cells and did not accumulate within characteristic BFA compartments. Latrunculin B-mediated depolymerization of F-actin inhibited internalization and accumulation of cell wall pectins within intracellular BFA compartments. Importantly, cold treatment and protoplasting prevented internalization of wall pectins into root cells upon BFA treatment. These observations suggest that cell wall pectins of meristematic maize root cells undergo rapid endocytosis in an F-actin-dependent manner.