Animal and plant cytokineses appear morphologically distinct. Recent studies, however, have revealed that these cellular processes have many things in common, including the requirement of co-ordinated membrane trafficking and cytoskeletal dynamics. At the intersection of these two processes are the members of the dynamin family of ubiquitous eukaryotic GTPases. In this review, we highlight the conserved contribution of classical dynamin and dynamin-related proteins during cytokinesis in both animal and plant systems. Cytokinesis is the final stage of the cell cycle in which a single cell is physically separated into individual daughter cells. In eukaryotes, this process leads to the division and partitioning of chromatin, organelles, cytoplasmic components and the construction of new membrane between daughter cells. Animal cells divide by constricting plasma membrane and targeting membrane along the newly formed cleavage furrow. The cleavage furrow compacts and bundles the microtubules (MTs) found in the spindle midzone into a structure called the midbody (1). On the other hand, cells of higher plants divide by constructing a unique cytoskeletal apparatus, the phragmoplast, across the division plane and targeting new plasma membrane and cell wall components to the center of the cell creating an intermediate membranous network called the cell plate (2). It has long been thought that the mechanisms of cytokinesis in animals and plants were quite different. Recent work, however, has uncovered highly conserved roles for several membrane trafficking and cytoskeletalassociated proteins in these seemingly different modes of cytokinesis (1). Dynamin and dynamin-related proteins have been shown to play essential roles in cell division in plants (3), animals (4,5) and the protist, Dictyostelium discoideum (6). The phylogenetic relationship of Dictyostelium to plant and animal species (7) likely suggests that there is an ancestral role for dynamin in cytokinesis. In this review, we highlight several studies that suggest that dynamin and dynamin-related proteins (DRPs) have conserved functions in a variety of cell division events.

Cell polarity is crucial for the generation of cell diversity. Recent evidence suggests that the actin cytoskeleton plays a key role in establishment of embryonic polarity, yet the mechanisms that maintain polarity cues in particular membrane domains during development remain unclear. Dynamin, a large GTPase, functions in both endocytosis and actin dynamics. Here, the C. elegans dynamin ortholog, DYN-1, maintains anterior polarity cues. DYN-1-GFP foci are enriched in the anterior cortex in a manner dependent on the anterior polarity proteins PAR-6 and PKC-3. Membrane internalization and actin comet formation are enriched in the anterior, and are dependent on DYN-1. PAR-6-labeled puncta are also internalized from cortical accumulations of DYN-1-GFP. Our results demonstrate a mechanism for the spatial and temporal regulation of endocytosis in the anterior of the embryo, contributing to the precise localization and maintenance of polarity factors within a dynamic plasma membrane.

Multicellular development requires the correct spatial and temporal regulation of cell division and differentiation. These processes are frequently coordinated by the activities of various signaling pathways such as Notch signaling. From a screen for modifiers of Notch signaling in Drosophila we have identified the RNA helicase Belle, a recently described component of the RNA interference pathway, as an important regulator of the timing of Notch activity in follicle cells. We found that loss of Belle delays activation of Notch signaling, which results in delayed follicle cell differentiation and defects in the cell cycle. Because mutations in well-characterized microRNA components phenocopied the Notch defects observed in belle mutants, Belle might be functioning in the microRNA pathway in follicle cells. The effect of loss of microRNAs on Notch signaling occurs upstream of Notch cleavage, as expression of the constitutively active intracellular domain of Notch in microRNA-defective cells restored proper activation of Notch. Furthermore, we present evidence that the Notch ligand Delta is an important target of microRNA regulation in follicle cells and regulates the timing of Notch activation through cis inhibition of Notch. Here we have uncovered a complex regulatory process in which the microRNA pathway promotes Notch activation by repressing Delta-mediated inhibition of Notch in follicle cells.

SUMMARYThe sensitivity of the posterior crossvein in the pupal wing of Drosophila to reductions in the levels and range of BMP signaling has been used to isolate and characterize novel regulators of this pathway. We show here that crossveinless d (cv-d) mutations, which disrupt BMP signaling during the development of the posterior crossvein, mutate a lipoprotein that is similar to the vitellogenins that comprise the major constituents of yolk in animal embryos. Cv-d is made in the liver-like fat body and other tissues, and can diffuse into the pupal wing via the hemolymph. Cv-d binds to the BMPs Dpp and Gbb through its Vg domain, and to heparan sulfate proteoglycans, which are well-known for their role in BMP movement and accumulation in the wing. Cv-d acts over a long range in vivo, and does not have BMP co-receptor-like activity in vitro. We suggest that, instead, it affects the range of BMP movement in the pupal wing, probably as part of a lipid-BMP-lipoprotein complex, similar to the role proposed for the apolipophorin lipid transport proteins in Hedgehog and Wnt movement.

No abstract