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.