An ABC transporter B family protein, ABCB19, is required for cytoplasmic streaming and gravitropism of the inflorescence stems

A significant feature of plant cells is the extensive motility of organelles and the cytosol, which was originally defined as cytoplasmic streaming. We suggested previously that a three-way interaction between plant-specific motor proteins myosin XIs, actin filaments, and the endoplasmic reticulum (ER) was responsible for cytoplasmic streaming. (1) Currently, however, there are no reports of molecular components for cytoplasmic streaming other than the actin-myosin-cytoskeleton and ER-related proteins. In the present study, we found that elongated cells of inflorescence stems of Arabidopsis thaliana exhibit vigorous cytoplasmic streaming. Statistical analysis showed that the maximal velocity of plastid movements is 7.26 µm/s, which is much faster than the previously reported velocities of organelles. Surprisingly, the maximal velocity of streaming in the inflorescence stem cells was significantly reduced to 1.11 µm/s in an Arabidopsis mutant, abcb19-101, which lacks ATP BINDING CASSETTE SUBFAMILY B19 (ABCB19) that mediates the polar transport of the phytohormone auxin together with PIN-FORMED (PIN) proteins. Polar auxin transport establishes the auxin concentration gradient essential for plant development and tropisms. Deficiency of ABCB19 activity eventually caused enhanced gravitropic responses of the inflorescence stems and abnormally flexed inflorescence stems. These results suggest that ABCB19-mediated auxin transport plays a role not only in tropism regulation, but also in cytoplasmic streaming.

Arabidopsis actin-depolymerizing factor7 severs actin filaments and regulates actin cable turnover to promote normal pollen tube growth

Actin filaments are often arranged into higher-order structures, such as the longitudinal actin cables that generate the reverse fountain cytoplasmic streaming pattern present in pollen tubes. While several actin binding proteins have been implicated in the generation of these cables, the mechanisms that regulate their dynamic turnover remain largely unknown. Here, we show that Arabidopsis thaliana actin-depolymerizing factor7 (ADF7) is required for turnover of longitudinal actin cables. In vitro biochemical analyses revealed that ADF7 is a typical ADF that prefers ADP-G-actin over ATP-G-actin. ADF7 inhibits nucleotide exchange on actin and severs filaments, but its filament severing and depolymerizing activities are less potent than those of the vegetative ADF1. ADF7 primarily decorates longitudinal actin cables in the shanks of pollen tubes. Consistent with this localization pattern, the severing frequency and depolymerization rate of filaments significantly decreased, while their maximum lifetime significantly increased, in adf7 pollen tube shanks. Furthermore, an ADF7-enhanced green fluorescent protein fusion with defective severing activity but normal G-actin binding activity could not complement adf7, providing compelling evidence that the severing activity of ADF7 is vital for its in vivo functions. These observations suggest that ADF7 evolved to promote turnover of longitudinal actin cables by severing actin filaments in pollen tubes.

A perspective on the mobilization, localization and delivery of molecules in the crowded bacterial cytoplasm

It has been assumed that diffusion of molecules in the bacterial cytoplasm is the mechanism that moves molecules in the absence of cytoplasmic streaming. However, is there an undiscovered mechanism present that mobilizes cytoplasm and its molecular contents, and delivers tRNAs to specific ribosomes at specific bacterial cytoplasmic locations? Mobilization of specific tRNA (and also mRNA transcripts and ribosomes) and cell division proteins to specific intracellular locations may suggest that instructions and/or mechanism(s) are needed. The alternative is that molecular crowding in the cytoplasm is sufficient for gentle contact between mRNA, ribosomes and tRNA. Or is it plausible that the bacterial cytoplasm (and its contents) are mobilized with the outcome being more gentle collisions between molecules than by a diffusion only mechanism? One hypothesis is that cytoplasmic and molecule mobilization and spatial organization are possibly driven by the photons in thermal infrared (IR) radiation and generation of exclusion zone (EZ) water in the cytoplasm.

The WAVE2/Abi1 complex differentially regulates megakaryocyte development and spreading: implications for platelet biogenesis and spreading machinery

Actin polymerization is crucial in throm-bopoiesis, platelet adhesion, and mega-karyocyte (MK) and platelet spreading. The Wiskott-Aldrich syndrome protein (WASp) homolog WAVE functions downstream of Rac and plays a pivotal role in lamellipodia formation. While MKs and platelets principally express WAVE1 and WAVE2, which are associated with Abi1, the physiologic significance of WAVE isoforms remains undefined. We generated WAVE2−/− embryonic stem (ES) cells because WAVE2-null mice die by embryonic day (E) 12.5. We found that while WAVE2−/− ES cells differentiated into immature MKs on OP9 stroma, they were severely impaired in terminal differentiation and in platelet production. WAVE2−/− MKs exhibited a defect in peripheral lamellipodia on fibrinogen even with phorbol 12-myristate 13-acetate (PMA) costimulation, indicating a requirement of WAVE2 for integrin αIIbβ3-mediated full spreading. MKs in which expression of Abi1 was reduced by small interfering RNA (siRNA) exhibited striking similarity to WAVE2−/− MKs in maturation and spreading. Interestingly, the knockdown of IRSp53, a Rac effector that preferentially binds to WAVE2, impaired the development of lamellipodia without affecting proplatelet production. In contrast, thrombopoiesis in vivo and platelet spreading on fibrinogen in vitro were intact in WAVE1-null mice. These observations clarify indispensable roles for the WAVE2/Abi1 complex in αIIbβ3-mediated lamellipodia by MKs and platelets through Rac and IRSp53, and additionally in thrombopoiesis independent of Rac and IRSp53.

Mutant huntingtin aggregates impair mitochondrial movement and trafficking in cortical neurons

Huntington's disease (HD) is a neurodegenerative disorder caused by a polyglutamine repeat in the huntingtin gene (Htt). Mitochondrial defects and protein aggregates are characteristic of affected neurons. Recent studies suggest that these aggregates impair cellular transport mechanisms by interacting with cytoskeletal components and molecular motors. Here, we investigated whether mutant Htt alters mitochondrial trafficking and morphology in primary cortical neurons. We demonstrate that full-length mutant Htt was more effective than N-terminal mutant Htt in blocking mitochondrial movement, an effect that correlated with its heightened expression in the cytosolic compartment. Aggregates impaired the passage of mitochondria along neuronal processes, causing mitochondria to accumulate adjacent to aggregates and become immobilized. Furthermore, mitochondrial trafficking was reduced specifically at sites of aggregates while remaining unaltered in regions lacking aggregates. We conclude that in cortical neurons, an early event in HD pathophysiology is the aberrant mobility and trafficking of mitochondria caused by cytosolic Htt aggregates.

Role of the spindle-pole-body protein ApsB and the cortex protein ApsA in microtubule organization and nuclear migration in Aspergillus nidulans

Nuclear migration and positioning in Aspergillus nidulans depend on microtubules, the microtubule-dependent motor protein dynein, and auxiliary proteins, two of which are ApsA and ApsB. In apsA and apsB mutants nuclei are clustered and show various kinds of nuclear navigation defects, although nuclear migration itself is still possible. We studied the role of several components involved in nuclear migration through in vivo fluorescence microscopy using fluorescent-protein tagging. Because ApsA localizes to the cell cortex and mitotic spindles were immobile in apsA mutants, we suggest that astral microtubule-cortex interactions are necessary for oscillation and movement of mitotic spindles along hyphae, but not for post-mitotic nuclear migration. Mutation of apsA resulted in longer and curved microtubules and displayed synthetic lethality in combination with the conventional kinesin mutation DeltakinA. By contrast, ApsB localized to spindle-pole bodies (the fungal centrosome), to septa and to spots moving rapidly along microtubules. The number of cytoplasmic microtubules was reduced in apsB mutants in comparison to the wild type, indicating that cytoplasmic microtubule nucleation was affected, whereas mitotic spindle formation appeared normal. Mutation of apsB suppressed dynein null mutants, whereas apsA mutation had no effect. We suggest that nuclear positioning defects in the apsA and apsB mutants are due to different effects on microtbule organisation. A model of spindle-pole body led nuclear migration and the roles of dynein and microtubules are discussed.

MgcRacGAP regulates cortical activity through RhoA during cytokinesis

Although Rho GTPases regulate multiple cellular events, their role in cell division is still obscure. Here we show that expression of a GTPase-activating protein (GAP)-deficient mutant (R386A) of the Rho regulator MgcRacGAP induces abnormal cortical activity during cytokinesis in U2OS cells. Multiple large blebs were observed in cells expressing MgcRacGAP R386A from the onset of anaphase to the late stage of cell division. When mitotic blebbing was excessive, cytokinesis was inhibited, and cells with micronuclei were generated. It has been reported that blebbing is caused by abnormal cortical activity. The MgcRacGAP R386A-induced abnormal cortical activity was inhibited by the dominant negative form of RhoA, but not Rac1 or Cdc42. Moreover, expression of constitutively active RhoA also induced drastic cortical activity during cytokinesis. Unlike apoptotic blebbing, MgcRacGAP R386A-induced blebbing was not inhibited by the ROCK inhibitor Y-27632, suggesting that MgcRacGAP regulates cortical activity during cytokinesis through a novel signaling pathway. We propose that MgcRacGAP plays a pivotal role in cytokinesis by regulating cortical movement through RhoA.

Programmed cell death remodels lace plant leaf shape during development

Programmed cell death (PCD) functions in the developmental remodeling of leaf shape in higher plants, a process analogous to digit formation in the vertebrate limb. In this study, we provide a cytological characterization of the time course of events as PCD remodels young expanding leaves of the lace plant. Tonoplast rupture is the first PCD event in this system, indicated by alterations in cytoplasmic streaming, loss of anthocyanin color, and ultrastructural appearance. Nuclei become terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling positive soon afterward but do not become morphologically altered until late stages of PCD. Genomic DNA is fragmented, but not into internucleosomal units. Other cytoplasmic changes, such as shrinkage and degradation of organelles, occur later. This form of PCD resembles tracheary element differentiation in cytological execution but requires unique developmental regulation so that discrete panels of tissue located equidistantly between veins undergo PCD while surrounding cells do not.

Contrasting patterns of mitochondrial redistribution in the early lineages of Caenorhabditis elegans and Acrobeloides sp. PS1146

We compared the redistribution of mitochondria in the early embryos of Caenorhabditis elegans (C. elegans) and Acrobeloides sp. PS1146 (Acrobeloides)--two nematode species where the mechanisms for embryonic axis specification are different even though subsequent development is remarkably similar. During the first cell cycle of C. elegans, mitochondria move with the bulk cytoplasmic flows that are directed toward the sperm pronucleus and aggregate at the posterior cortex during the period known as "pseudocleavage." In contrast, in Acrobeloides embryos, where prominent cytoplasmic rearrangements are absent, mitochondria that are initially distributed loosely around the pronuclei and the cytoplasm are relocated around the mitotic spindle prior to cell division. Interestingly, this rearrangement is reiterated only in the germline and not the somatic lineage. In both species, the location of the mitochondria immediately prior to cell division correlates with the known location of the germline determinants, P granules, leading us to speculate that they may be associated.

Nucleocytoplasmic transport of HTLV-1 RNA is regulated by two independent LTR encoded nuclear retention elements

Appropriate expression of HTLV-1 genes requires transcriptional transactivation by Tax and post-transcriptional regulation by Rex, both mediated by LTR encoded RNA sequences. Using a combination of deletion mutagenesis, Rex-reporter CAT assays, fluorescence in situ hybridization (FISH) and confocal laser scanning microscopy it was established that in the absence of Rex, CAT mRNAs harboring HTLV-1 LTR sequences were unable to leave the nucleus. Deletion of the known U5 encoded cis-acting repressing sequence (CRS) led to a partial release of nuclear retention. A novel regulatory element overlapping the 3' Rex responsive element (RxRE) region was shown to prevent export and expression of these transcripts. Deletion of both the 5' LTR encoded CRS and 3' LTR encoded downstream repressive sequence (3' CRS) led to constitutive mRNA nuclear export and gene expression, independently of Rex. The locations of the two regulatory elements indicate that while the 5' CRS selectively acts to hinder export of unspliced transcripts, the 3' CRS has the capacity to induce nuclear retention of all HTLV-1 transcripts, and therefore could potentially contribute to viral latency in infected cells.

The myosin I SH3 domain and TEDS rule phosphorylation site are required for in vivo function

The class I myosins play important roles in controlling many different types of actin-based cell movements. Dictyostelium cells either lacking or overexpressing amoeboid myosin Is have significant defects in cortical activities such as pseudopod extension, cell migration, and macropinocytosis. The existence of Dictyostelium null mutants with strong phenotypic defects permits complementation analysis as a means of exploring important functional features of the myosin I heavy chain. Mutant Dictyostelium cells lacking two myosin Is exhibit profound defects in growth, endocytosis, and rearrangement of F-actin. Expression of the full-length myoB heavy chain in these cells fully rescues the double mutant defects. However, mutant forms of the myoB heavy chain in which a serine at the consensus phosphorylation site has been altered to an alanine or in which the C-terminal SH3 domain has been removed fail to complement the null phenotype. The wild-type and mutant forms of the myoB heavy chain appeared to be properly localized when they were expressed in the myosin I null mutants. These results suggest that the amoeboid myosin I consensus phosphorylation site and SH3 domains do not play a role in the localization of myosin I, but are absolutely required for in vivo function.