Spatial effects - site-specific regulation of actin and microtubule organization by septin GTPases

Abstract

The actin and microtubule cytoskeletons comprise a variety of networks with distinct architectures, dynamics and protein composition. A fundamental question in eukaryotic cell biology is how these networks are spatially and temporally controlled, so they are positioned in the right intracellular places at the right time. While significant progress has been made in understanding the self-assembly of actin and microtubule networks, less is known about how they are patterned and regulated in a site-specific manner. In mammalian systems, septins are a large family of GTP-binding proteins that multimerize into higher-order structures, which associate with distinct subsets of actin filaments and microtubules, as well as membranes of specific curvature and lipid composition. Recent studies have shed more light on how septins interact with actin and microtubules, and raised the possibility that the cytoskeletal topology of septins is determined by their membrane specificity. Importantly, new functions have emerged for septins regarding the generation, maintenance and positioning of cytoskeletal networks with distinct organization and biochemical makeup. This Review presents new and past findings, and discusses septins as a unique regulatory module that instructs the local differentiation and positioning of distinct actin and microtubule networks.

Keywords: Actin; Actin microtubule patterning; Membrane-cytoskeleton crosstalk; Microtubules; Rho signaling; Septins; Spatial organization and regulation.

Fig. 1.

The roles of septin in actin SF organization. (A) Confocal images showing a fibroblast cell (NIH 3T3) stained with phalloidin (actin; black and white) and anti-SEPT7 (green). SEPT7 decorates partially a subset of SFs and is largely absent from lammelipodia. Scale bar: 5 µm. (B) Structured illumination super-resolution image shows a peripheral lamellar region from a motile MDCK cell stained with phalloidin (actin; red), anti-SEPT2 (green) and anti-paxillin (white). Arrows point to SEPT2 localization at transverse arc SFs and their junctions with dorsal (radial) SFs. Scale bar: 5 µm. (C) Model of how septins may coordinate actin SF interactions at the peripheral lamellae of motile cells. Septins associate with the distal ends of FA-anchored dorsal (radial) SFs and may mediate their anchoring to transverse arc SFs (left). In parallel, septins may promote the generation of ventral SFs by annealing transverse arc SFs with the free ends of dorsal (radial) SFs from two opposing FAs (right).

Fig. 2.

Septins crosslink MTs into bundles, and regulate MT polyglutamylation and acetylation. (A) Confocal image shows SEPT9 (red) localization to a subset of MTs (grayscale) in MDCK cells. Scale bar: 10 µm. (B) Illustration of the crosslinking of MTs into elongated bundles by SEPT9-containing complexes (red). The N-terminal MT-binding domain (MTBD) of SEPT9 consists of repeat motifs that interact electrostatically with the C-terminal tails of β-tubulin. (C) Model for the regulation of MT polyglutamylation through MT-associated septins based on recent findings in taxane-resistant cancer cells. (Left) Binding of septins to MTs is promoted by the C-terminal Tyr residue (Y) of α-tubulin (α) and side-chains of 1−3 Glu residues (E). β, β-tubulin. (Middle) Upon association with MTs, septins are thought to scaffold the recruitment of the enzymes TTLL1 and TTLL11, which elongate the glutamate chains of α-tubulin. (Right) Concomitantly, septins interact with CCP1, which trims the length of the polyglutamate chains. (D) Deacetylation of cytosolic α-tubulin requires the interaction of HDAC6 with SEPT7. Cytosolic septin complexes that contain SEPT7 are thought to provide a scaffold for the deacetylation of non-polymerized tubulin.

Fig. 3.

The cytoskeletal topology of septins might be determined by their membrane-binding properties. (A) Septin accumulation to peripheral lamellar regions with transverse arc SFs are in part dependent on PI(3,5)P2-containing endolysosomes. LE, late endosome. (B) Septin association with ER-derived phosphatidylinositol 5-phosphate (PI5P)-containing lipid droplets may result in septin binding to MT subsets that are spatially proximal to these membranes. (C) Septins associate preferentially with plasma membrane domains that have a positive curvature at the micron-scale, e.g. domains that outline the neck and base of protrusive structures, such as filopodia, lammelipodia and dendritic spines. At these locations, septins might crosslink actin filaments and, possibly, capture or guide MTs.