SummaryUsing a bacterial two-hybrid system and a combination of in vivo and in vitro assays that take advantage of the green fluorescent reporter protein (GFP), we have investigated the localization and the proteinprotein interaction of several key components of the cytokinetic machinery of cyanobacteria (i.e. the progenitor of chloroplast). We demonstrate that (i) the ftsZ and zipN genes are essential for the viability of the model cyanobacterium Synechocystis sp. PCC 6803, whereas the minCDE cluster is dispensable for cell growth; (ii) the GTP-binding domain of FtsZ is crucial to FtsZ assembly into the septal ring at midcell; (iii) the Z-ring of deeply constricted daughter cells is oriented perpendicularly to the mother Z-ring, showing that Synechocystis divides in alternating perpendicular planes; (iv) the MinCDE system affects the morphology of the cell, as well as the position and the shape of FtsZ structures; and (v) MinD is targeted to cell membranes in a process involving its Cterminal amphipathic helix, but not its ATP-binding region. Finally, we have also characterized a novel Zinteracting protein, ZipN, the N-terminal DnaJ domain of which is critical to the decoration of the Z-ring, and we report that this process is independent of MinCDE.

Numb acts as a cell-fate determinant during asymmetric and stem cell divisions in both vertebrates and invertebrates [1, 2]. In Drosophila, Numb is unequally segregated in asymmetrically dividing sensory organ precursor cells (SOPs). Numb is inherited by the pIIb cell (Notch OFF) and is absent from the pIIa cell (Notch ON) [3, 4]. Numb is required to establish directional Notch signaling during cytokinesis [3, 5-7]. Using real-time imaging of a functional GFP-tagged Numb, we show that Numb relocalizes during cytokinesis from the basal cortex of pIIb to subapical endosomes. This relocalization appeared to depend on its interaction with the α-adaptin [8, 9]. Live imaging of Sanpodo (Spdo), a membrane protein interacting with Numb and regulating the trafficking of Notch [6, 7, 10-15], revealed that Spdo is internalized during cytokinesis and coaccumulates with Numb in pIIb endosomes. Using a GFP-tagged Notch [6], we found that Notch coaccumulates with Spdo in a Numb-dependent manner in these pIIb endosomes. Numb was, however, dispensable for the internalization of Notch and Spdo. We propose that Numb interacts with internalized Spdo-Notch oligomers at sorting endosomes and inhibits the recycling of Notch, thereby creating an asymmetry in Notch distribution along the pIIa-pIIb interface and regulating binary fate choice.

The emergence of spatial patterns in developing multicellular organisms relies on positional cues and cell-cell communication. sensory organs have informed a paradigm in which these operate in two distinct steps: Prepattern factors drive localized proneural activity, then Notch-mediated lateral inhibition singles out neural precursors. Here we show that self-organization through Notch signaling also establishes the proneural stripes that resolve into rows of sensory bristles on the fly thorax. Patterning, initiated by a gradient of Delta ligand expression, progresses through inhibitory signaling between and within stripes. Thus, Notch signaling can support self-organized tissue patterning as a prepattern is transduced by cell-cell interactions into a refined arrangement of cellular fates.