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First published online December 21, 2005
doi: 10.1242/10.1242/jcs.02746

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Here come the septins: novel polymers that coordinate intracellular functions and organization

Departments of Biological Sciences, and Molecular and Cellular Physiology, Stanford University, Stanford, CA 94305, USA

View larger version (85K): [in a new window]   Fig. 1. Mammalian septins localize to the plasma membrane and with the actin and microtubule cytoskeletons. (A,B) SEPT2 localizes to the cell cortex, the microtubule spindle apparatus and the midbody of mammalian cells (MDCKs) in mitosis. (C) SEPT2 (red) localizes to membranes (green) labeled with gpiYFP (glycosylphosphatidylinositol linked to yellow fluorescence protein) in interphase MDCK cells. The outlined area shows a region of cell-cell contact in which SEPT2 elements are closely apposed to the plasma membrane. (D) SEPT2 elements (green) colocalize with the microtubule (blue; arrow heads) and actin (red; arrows) cytoskeletons in interphase MDCK cells. All images represent 1 µm optical sections obtained by confocal microscopy. Bars, 10 µm.

View larger version (26K): [in a new window]   Fig. 2. (A) Septin structure. All mammalian septins consist of a polybasic region and a GTP-binding domain of the P-loop superfamily of GTPases. Their N- and C-terminal regions vary in length and amino acid composition, and contain proline-rich (red) and -helical coiled-coil (green) domains. (B) Assembly and classification of mammalian septins based on amino acid similarity (Kartmann and Roth, 2001; Kinoshita, 2003; Hall et al., 2005). Septins are classified into four groups: SEPT3 (SEPT3, SEPT9 and SEPT12), SEPT2 (SEPT2, SEPT1, SEPT4 and SEPT5), SEPT7 (SEPT7 and SEPT13) and SEPT6 (SEPT6, SEPT8, SEPT10 and SEPT11). Note that proteins of the SEPT7 group were recently classified under the SEPT2 group (Hall et al., 2005). Proteins of the SEPT3 group lack C-terminal coiled-coil domains, and the SEPT2 group contains a shorter coiled-coil region than SEPT6. Septins bind one another through their coiled-coil domains to form hetero-oligomers and hetero-polymers (Sheffield et al., 2003). SEPT9 isoforms (SEPT3 group) associate with other septins through their N-terminal domain (Nagata et al., 2004).

View larger version (22K): [in a new window]   Fig. 3. Septins at the interface between cell signaling, and membrane and cytoskeleton biology. Mammalian septins assemble into hetero-polymers under the influence of the Rho and Cdc42 signaling modules. Phosphorylation of mammalian septins by protein kinases in neurons suggests that post-translational modifications (e.g. phosphorylation and sumoylation) might further modulate the dynamics of their assembly. Hence, septins can dynamically rearrange under the guidance of signaling pathways to support various intracellular functions, including organization and function of the actin and microtubule (MT) cytoskeletons, formation and maintenance of plasma membrane domains, and vesicle transport and fusion.