Septin structure and function in yeast and beyond

Abstract

Septins are conserved GTP-binding proteins that assemble into hetero-oligomeric complexes and higher-order structures such as filaments, rings, hourglasses or gauzes. Septins are usually associated with a discrete region of the plasma membrane and function as a cell scaffold or diffusion barrier to effect cytokinesis, cell polarity, and many other functions. Recent structural studies of septin complexes have provided mechanistic insights into septin filament assembly, but key questions concerning the assembly, dynamics, and function of different septin structures remain to be answered.

Fig. 1

Structures of septin complexes and higher-order assemblies in budding yeast and beyond. (a) Left: concentric rings of septin filaments in a septin hourglass visualized in a grazing section of the mother-bud neck (scanned from the original EM picture published in [1]; courtesy of B. Byers). CW, cell wall; PM, plasma membrane. Right: higher magnification of a portion of the image at the left showing diagonal short filaments (parallel to the black line) involved in connecting the rings of septin filaments in the hourglass (reproduced from [1] by copyright permission). (b) Left: septin filaments of an hourglass visualized in a transverse section of the mother-bud neck showing close association of the filaments with the PM (scanned from the original EM picture published in [1]; courtesy of B. Byers). M, mitochondrion. Right: higher magnification of a portion of the image at the left showing interactions of the septin filaments with themselves and the PM. f, filaments; lc, lateral connections; mc, membrane connections (reproduced from [1] by copyright permission). (c) A portion of the septin “gauze” (presumably from the hourglass structure at the mother-bud neck) from a piece of isolated cell cortex visualized by rapid-freeze and deep-etch EM (reproduced from [17] with permission). Arrow, long paired filaments. A single pair of filaments is highlighted in red. (d) Motifs of a generic septin molecule. PB, polybasic region; G domain, guanine nucleotide binding domain; CC, coiled-coil region; N and C, N and C termini of a septin. (e) Native septin complexes (High salt) and their assembled paired filaments (Low salt) visualized by negative-stain EM (reproduced from [12] by copyright permission). (f) Left: septin subunit arrangement in the recombinant septin complex Cdc3/Cdc10/Cdc11/Cdc12 (reproduced from [15] with permission). Right: comparison of the structures of the septin complexes from budding yeast (top), human (middle), and C. elegans (bottom). Blue and red rectangle boxes, the G domain of a septin; blue and red coils, the coiled-coil region of a septin; N and C, the N- and C-termini of a septin. The guanine nucleotides (GDP or GTP) bound in the mammalian, but not the budding yeast and C. elegans, septin complexes are known and thus indicated. (g) Models for the arrangements of septin filaments in the septin hourglass (the “septin-gauze” model) and the split septin rings. Top: the septin hourglass (see the enlarged boxed area) is chiefly made of paired septin filaments (vertically arrayed double red lines) in the form of concentric rings or a spiral that are inter-connected by short filaments (diagonally arrayed thin red lines) as well as some long paired septin filaments (horizontal double red lines). M, mother; D, daughter; Blue circle, nucleus. Bottom: the split septin rings (see the enlarged boxed area) consist of an array of short septin filaments (red lines) in parallel to the long mother-daughter axis.

Fig. 2

Septin organization and dynamics during the cell cycle in budding yeast. After the launch of a new cell cycle in late G1, a septin ring is assembled at the presumptive bud site (0 min). This nascent septin ring is dynamic by FRAP analysis. Upon bud emergence in S phase, the septin ring is converted into a stable hourglass structure, which is maintained at the bud neck during polarized bud growth from S to late anaphase (27 and 81 min). Around the onset of cytokinesis in telophase, the septin hourglass is split into two dynamic cortical rings sandwiching the division machinery (114 min). The septin rings remain at the old division sites for both the mother and the daughter cells until the next round of budding when the “old rings” are disassembled while the new rings are assembled at the presumptive bud site. The transitions in septin organization during the cell cycle are regulated by CDK/cyclins, protein kinases and phosphatases etc (see text for details). Image acquisition and processing for this figure (courtesy of S. Okada): a wild-type haploid cell carrying GFP-tagged Cdc3 was analyzed by 3D spin-disk time-lapse microscopy (20 Z-sections of 0.3 - μm were collected for each time point; interval, 3 - min). The images shown above are snap-shots of 3D reconstructions of the cell from selected time points of a time-lapse series and at an appropriate angle of rotation to show septin structures of interest.

Fig. 3

A model for Cdc42-controlled septin ring assembly in budding yeast. Budding yeast undergoes asymmetric division by default, which means that at the time of cytokinesis and cell separation, the daughter cell (D, 0 min) is substantially smaller than the mother cell (M, 0 min). Consequently, the mother cell enters a new cell cycle immediately after cell division whereas the daughter cell stays in G1 for longer period to reach a critical cell mass required for the launch of a new cell cycle. Thus, in this figure, only the septin ring assembly at the mother side is depicted. We propose that septin ring assembly at the presumptive bud site (PBS, 0 min) involves three steps: septin recruitment and membrane association, ring assembly, and ring maturation (see text for details). Single arrows, the PBS and the septin structures of interest (“septin cloud”, 24 min; “septin ring”, 36 min; “septin hourglass”, 78 min). Image acquisition and processing for this figure are the same as described for Figure 2.