Self- and Actin-Templated Assembly of Mammalian Septins

Abstract

Septins are polymerizing GTPases required for cytokinesis and cortical organization. The principles by which they are targeted to, and assemble at, specific cell regions are unknown. We show that septins in mammalian cells switch between a linear organization along actin bundles and cytoplasmic rings, approximately 0.6 microm in diameter. A recombinant septin complex self-assembles into rings resembling those in cells. Linear organization along actin bundles was reconstituted by adding an adaptor protein, anillin. Perturbation of septin organization in cells by expression of a septin-interacting fragment of anillin or by septin depletion via siRNA causes loss of actin bundles. We conclude that septins alone self-assemble into rings, that adaptor proteins recruit septins to actin bundles, and that septins help organize these bundles.

Some assembly required: yeast septins provide the instruction manual

“…All septin complexes isolated from mammalian cells also contain either one or two additional septins: one from the Sept2 group (Sept1, Sept2, Sept4 and Sept5), the ortholog of which in the fly is Sep1; and/or, one from the Sept3 group (Sept3, Sept9 and Sept12), which, as mentioned previously, lack a CTE, like yeast Cdc10 (Spn2). Moreover, the apparent stoichiometry and molecular weight of all septin complexes from animal cells are most consistent with the presence of two copies of each subunit [7,9,45]. Based on sequence similarities (if clear-cut), the analogies in composition of the complexes, and the interrelationships discussed above, we propose that septins can be classified into conserved groups (Table 1) and that similar models for the corresponding complexes can be deduced (Figure 1b).…”

“…Filament formation increases in low-salt conditions [7][8][9]. In budding yeast [10], this effect has been attributed to the salt-sensitive nature of the interaction between Cdc3 and Cdc11, an interface that is pivotal to polymer formation ( Figure 2).…”

“…Similarly, in fission yeast, the band of septin filaments that assembles at the medial cortex splits into two rings as the septum forms [19,20,32]. In mammalian cells, actin fibers act as templates for septin filaments, and when actin is perturbed in vivo, coils and rings of septins result [3,9,49]. In addition, SNARE proteins and microtubules are reported to contribute to the localization and/or assembly of septin filaments in animal cells [23,[50][51][52].…”

“…With the notable exception of Xenopus laevis Sept2 [55], single septins and complexes that are composed of two different septins are not reported to polymerize in vitro. In all other cases examined, the capacity to polymerize into filaments requires the presence of complexes that contain at least three types of septin, for example, Cdc3-Cdc12-Cdc11 in budding yeast [10], Spn1-Spn4-Spn3 and Spn1-Spn4-Spn2 in fission yeast [33], Sept6-Sept7-Sept2 in mammalian cells [9,45] and Sep2-Pnut-Sep1 in flies [7]. Although the Cdc3-Cdc12-Cdc11-containing complex polymerizes into filaments in vitro, Cdc10 makes crucial contributions to septin-filament organization and stability in budding yeast (Box 2) [10], and Cdc10 counterparts in other organisms presumably serve similar functions.…”

“…Septin monomers assemble into hetero-oligomeric (multi-septin) complexes (models based on the available evidence are shown in Figure 1b). The complexes polymerize into filaments in vitro (Figure 2a) [7][8][9][10] and in vivo (Figure 2b) [11][12][13]. A model of filaments in S. cerevisiae is shown in Figure 2c.…”