Septins are evolutionary conserved
proteins with essential functions in cytokinesis, and more subtle roles throughout the cell cycle. Much of our knowledge about septins originates from studies of budding yeast" Saccharomyces cerevisiae," where they form a ring-like proteinscaffold at the mother-bud neck.
eptins in "Saccharomyces cerevisiae"
The septins were discovered in 1970 by
Leland H. Hartwelland colleagues in a screen for temperature-sensitive mutants affecting cell division (cdc mutants). The screen revealed four mutants which prevented cytokinesisat restrictive temperature. The corresponding genes represent the four original septins, "ScCDC3, ScCDC10, ScCDC11," and "ScCDC12". Despite disrupted cytokinesis, the cells continued budding, DNA synthesis, and nuclear division, which resulted in large multinucleatecells with multiple, elongated buds. In 1976, analysis of electron micrographs revealed ~20 evenly spaced striations of 10-nm filaments around the mother-bud neck in wild-type but not in septin-mutant cells. Immunofluorescencestudies revealed that the septin proteins colocalize into a septin ringat the neck. The localization of all four septins is disrupted in conditional "Sccdc3" and "Sccdc12" mutants, indicating interdependence of the septin proteins. Strong support for this finding was provided by biochemical studies: The four original septins co-purified on affinity columns, together with a fifth septin protein, encoded by "ScSEP7" or "ScSHS1". Purified septins from budding yeast, Drosophila, Xenopus, and mammalian cells are able to self associate " in vitro" to form highly ordered, filamentous structures. How the septins interact " in vitro" to form heteropentamers that assemble into filaments was studied in detail in "S. cerevisiae". Based on these and former studies, the septins are composed of a variable N-terminuswith a basic phosphoinositidebinding motif, a conserved core comprising a GTP-binding domain, a septin-unique elementand a C-terminalextension including a predicted coiled coil. Micrographs of purified filaments raised the possibility that the septins are organized in parallel to the mother-bud axis. The 10-nm striations seen on electron micrographs may be the result of lateral interaction between the filaments. Mutantstrains lacking factors important for septin organization support this view. Instead of continuous rings, the septins form bars oriented along the mother-bud axis in deletion mutants of "ScGIN4, ScNAP1" and "ScCLA4".
The septins act as a scaffold, recruiting a plethora of
proteins. These protein complexes are involved in cytokinesis, chitindeposition, cell polarity, sporeformation, in the morphogenesischeckpoint, spindle alignment checkpoint and bud site selection.
cytokinesisis driven through two septin dependent, redundant processes: recruitment and contraction of the actomyosin ring and formation of the septum by vesicle fusion with the plasma membrane. In contrast to septin mutants, disruption of one single pathway only leads to a delay in cytokinesis, not complete failure of cell division. Hence, the septins are predicted to act at the most upstream level of cytokinesis.
apical-isotropic switch in budding yeast, cortical components, supposedly of the exocystand polarisome, are delocalized from the apical pole to the entire plasma membrane of the bud, but not the mother cell. The septin ringat the neck serves as a cortical barrier that prevents membrane diffusionof these factors between the two compartments. This asymmetric distribution is abolished in septin mutants.
Some conditional septin
mutants do not form buds at their normal axial location. Moreover, the typical localization of some bud-site-selection factors in a double ring at the neck is lost or disturbed in these mutants. This indicates that the septins may serve as anchoring site for such factors in axially buddingcells.
It seems that one single septin organization should not be sufficient to fulfill such a variety of tasks. Accordingly, the septin cortex undergoes several changes throughout the
cell cycle: The first visible septin structure is a distinct ring which appears ~15 min before bud emergence. After bud emergence, the ring broadens to assume the shape of an hourglassaround the mother-bud neck. During cytokinesis, the septin cortex splits into a double ring which eventually disappears. How can the septin cortex undergo so dramatic changes, although some of its functions may require it to be a stable structure? FRAP analysis has revealed that the turnover of septins at the neck undergoes multiple changes during the cell cycle. The predominant, functional conformation is characterized by a low turnover rate (frozen state), during which the septins are phosphorylated. Structural changes require a destabilization of the septin cortex (fluid state) induced by dephosphorylationprior to bud emergence, ring splitting and cell separation.
The composition of the septin cortex does not only vary throughout the
cell cyclebut also along the mother-bud axis. This inherent polarity of septin filaments allows concentration of some proteins primarily to the mother side of the neck, some to the center and others to the bud site.
eptins in filamentous fungi
Since their discovery in "S. cerevisiae," septin homologues have been found throughout the
eukaryotickingdom, with the exception of plants. The variety of different shapes that septins can assume within a single cell is especially apparent in filamentous fungi, where they control aspects of filamentous morphology.
genomeof "C. Albicans" encodes homologues to all "S. cerevisiae" septins ("CaCDC3, CaCDC10, CaCDC11, CaCDC12, CaSEP7"). They form a diffuse band at the base of emerging hyphae, a bright double ring at septationsites, an extended diffuse cap at hyphaltips and elongated filaments stretching around the spherical chlamydospores. As an effect of maturation, double rings reflect hyphalpolarity by disassembling the tip proximal ring. CaCdc3p and CaCdc12p are essential for proliferation in yeastor hyphalforms. "Cacdc10Δ "and "Cacdc11Δ "deletion mutants are viable but show aberrant chitinlocalization and cannot properly maintain hyphalgrowth direction.
Five septins are found in "A. nidulans" (AnAspAp, AnAspBp, AnAspCp, AnAspDp, AnAspEp). AnAspBp forms single rings at septation sites that eventually split into double rings. Additionally, AnAspBp forms a ring at sites of branch emergence which broadens into a band as the branch grows. Like in "C. Albicans," double rings reflect polarity of the
hypha, but by disassembling the more basal ring. Bases for the various patterns of septin organization could be different modifications and/or localization of different septin interaction partners. Conditional mutants of the essential AnAspBp display diffuse chitindeposition and a hyper-branching phenotype.
" "'(fluorescent micrograph)
• Green: septins ("AgSEP7-GFP")
• Red: cell outline (phase contrast)
• Inlay: 3D reconstruction of a discontinuous septin ring
• Scale bars: 10 μm] The "
ascomyceteA. gossypii" possesses homologues to all "S. cerevisiae" septins, with one being duplicated ("AgCDC3, AgCDC10, AgCDC11A, AgCDC11B, AgCDC12, AgSEP7"). " In vivo" studies of AgSep7p-GFP have revealed that septins assemble into discontinuous hyphalrings close to growing tips and sites of branch formation and into asymmetric structures at the base of branching points. Rings are made of filaments which are long and diffuse close to growing tips and short and compact further away from the tip. During septum formation, the septin ringsplits into two to form a double ring. "Agcdc3Δ, Agcdc10Δ "and "Agcdc12Δ "deletion mutants display aberrant morphology and are defective for actin-ring formation, chitin-ring formation, and sporulation. Due to the lack of septa, septin deletion mutants are highly sensitive, and damage of a single hyphacan result into complete lysisof a young mycelium.
*"The septins: roles in cytokinesis and other processes." Longtine, M.S., DeMarini, D.J., Valencik, M.L., Al-Awar, O.S., Fares, H., De Virgilio, C., and Pringle, J.R. (1996). Curr. Opin. Cell Biol. 8, 106-119.
*"The septin cortex at the yeast mother-bud neck." Gladfelter, A.S., Pringle, J.R., and Lew, D.J. (2001). Curr Opin Microbiol 4, 681-689.
*"Septins: a ring to part mother and daughter." Faty, M., Fink, M., and Barral, Y. (2002). Curr Genet 41, 123-131.
*"Protein-protein interactions governing septin heteropentamer assembly and septin filament organization in "Saccharomyces cerevisiae." Versele, M., Gullbrand, B., Shulewitz, M.J., Cid, V.J., Bahmanyar, S., Chen, R.E., Barth, P., Alber, T., and Thorner, J. (2004). Mol Biol Cell 15, 4568-4583.
*"Septin function in yeast model systems and pathogenic fungi." Douglas, L.M., Alvarez, F.J., McCreary, C., and Konopka, J.B. (2005). Eukaryot Cell 4, 1503-1512.
*"Control of filamentous fungal cell shape by septins and formins." Gladfelter, A.S. (2006). Nat Rev Microbiol 4, 223-229.
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