Spatial regulation of microtubule-dependent transport by septin GTPases

Abstract

The intracellular long-range transport of membrane vesicles and organelles is mediated by microtubule motors (kinesins, dynein) which move cargo with spatiotemporal accuracy and efficiency. How motors navigate the microtubule network and coordinate their activity on membrane cargo are fundamental but poorly understood questions. New studies show that microtubule-dependent membrane traffic is spatially controlled by septins - a unique family of multimerizing GTPases that associate with microtubules and membrane organelles. We review how septins selectively regulate motor interactions with microtubules and membrane cargo. We posit that septins provide a novel traffic code that specifies the movement and directionality of select motor-cargo complexes on distinct microtubule tracks.

Keywords: dynein; kinesins; microtubule-associated proteins; microtubules; scaffold and adaptor proteins; septins.

Figure 1.. Microtubule-associated Septins

(A) Septins interact directly with and crosslink microtubules into bundles. Septin interaction with microtubules involves the C-terminal tyrosine of α-tubulin, polyglutamylation of the C-terminal tails of tubulin and the C-terminal tail of β-tubulin. Septins associate preferentially with GTP- and taxol-bound microtubules, which are characterized by a non-compacted longitudinal inter-dimer interface. Yellow highlights the structural elements of α-/β-tubulin that play a role in septin-microtubule binding. (B) Septins function in the elongation and trimming of the polyglutamyl sidechains of the tubulin C-terminal tails by interacting with the polyglutamylase enzymes tubulin tyrosine ligase like 1 and 11 (TTLL1, TTLL11) and the cytosolic carboxypeptidase 1 (CCP1), which removes glutamyl residues. (C) Septins have been shown to bundle microtubules, suppress microtubule catastrophe and thereby, promote persistent microtubule growth and elongation. In addition, SEPT9 recruits unpolymerized tubulin dimers to the microtubule lattice, which might promote the incorporation of tubulin subunits at sites of microtubule damage.

Figure 2.. Membrane-associated Septins

Septins associate preferentially with membrane domains of micron-scale curvature (radius of 0.5–1.5 um) and phosphoinositide content that includes phosphatidylinositol 4-phosphate, PI4P, phosphatidylinositol 4,5-bisphosphate, PI(4,5)P2, phosphatidylinositol 3,5-bisphosphate, PI(3,5)P2, as well as cone-shaped lipids such as cardiolipin, phosphatidic acid (PA) and diacylglycerol (DAG), which favorably partition in membrane areas of curvature. Septins have been reported to localize on phagosomes, macropinosomes, lipid droplets, multivesicular bodies, lysosomes, mitochondria, bacteria, and Golgi membranes. SEPT2 interacts with the exocyst vesicle-tethering complex and the cytoplasmic tail of the glutamate aspartate receptor (GLAST). SEPT7 interacts with the Rab8 GTPase and the clathrin adaptor protein 3 (AP3). SEPT9 associates with the tumor suppressor gene 101, a component of the endosomal soring complex required for transport (ESCRT), and septins (SEPT2, SEPT7, SEPT9) are in a complex with the nexin SNX-21 and the adaptor protein huntingtin.

Figure 3.. Septin Interactions with Microtubule Motors and Functions in Mitosis

(A) Summary of direct (solid lines) and potentially indirect (dotted lines) interactions of septin paralogs with dynactin and dynein components, and kinesins KIF17, KIF20A and CENP-E. Dotted rectangles outline the C-terminal kinesin domains that interact directly with septins. (B) Cargo-bound septins recruit and scaffold motors of the same or opposite directionalities (KIF17 and dynein), providing a mechanism for the selective assembly of motor teams and possibly the regulation of bidirectional movement. (C) Septins (SEPT7) interact directly with CENP-E and function in proper chromosome alignment and bi-orientation at the metaphase plate by maintaining proper CENP-E localization. Septins may enhance processive motility of CENP-E, promoting CENP-E-mediated translocation of mono-oriented chromosomes along spindle microtubules toward the metaphase plate. At the metaphase plate, microtubule-associated septin complexes may scaffold CENP-E interactions with microtubule ends and kinetochores, and/or blocking the diffusion and movement of CENP-E away from the kinetochore-microtubule interface. In the latter scenario, microtubule-associated septins are posited to restrict CENP-E diffusion along the microtubule lattice. (D) A hypothetical model of septin roles in the localization and functions of KIF20A in the central spindle of mitotic cells. Septins localize to central spindle microtubules and may trigger the dissociation of CPC from KIF20A as SEPT7 interacts with the same domain of KIF20A, which associates with the INCENP component of the CPC. Additionally, microtubule-associated septins may immobilize KIF20A and promote its localization at the spindle midzone.

Figure 4.. A Septin GTPase Code for Motor Selection by Cargo and Microtubules

Graphical abstract of the septin code hypothesis for the selective attachment of motors to cargo (left panel) and motor-cargo binding and transport on microtubules (right panel). In the left panel, different microtubule motors (kinesins X, Y and dynein) are recruited to different cargos (cargos A, B and C) by cargo-specific septins or septin complexes, which are denoted by roman numerals (septins I, II, III). In the right panel, microtubule tracks with distinct septin complexes select for the attachment and motility of specific motor-cargo (cargo A-motor X, cargo B-motor Y, cargo C-dynein). Thereby, microtubule-associated septins specify the microtubule tracks and intracellular routes of specific motors and their cargo.

Box 1, Figure I.. Septin Domains and Assembly

(A) Schematic shows the main domains of septins, which include the highly conserved G-domain and SUE, and the variable NTE and CTE. The positions of the membrane-binding polybasic domains and amphipathic helices (blue), and the microtubule-binding repeat motifs (K/R-R/x-x-D/E; red) are also depicted. Microtubule-binding motifs are present in the NTE of SEPT9, but similar motifs may mediate the interaction of the CTEs of SEPT6 and SEPT7 with microtubules. (B) Schematic of the position and orientation of the major domains of septins and the GTP/GDP nucleotide. GTPase activity is denoted as minus (−) for septins of the SEPT6 group, which do not hydrolyze GTP, and plus (+) for septins with slower GTPase such as SEPT2 and SEPT7. SEPT9, which is depicted as the ubiquitously expressed septin paralog of the SEPT3 group, has faster rates (+++) of GTP hydrolysis than SEPT2 and SEPT7. (C) Schematic representation of the prototypical SEPT2/6/7/9 hetero-octamer, which consists of septin paralogs interacting with one another through two different bindings interfaces, which involve the GTP-binding pockets (G-G interface) and the N- and C-termini of the GTP-binding domains (NC-NC interface).

Box 2, Figure I.. Regulation of Kinesin and Dynein motility by the Microtubule-binding Domain of SEPT9

(A) Differential regulation of motor motility by microtubule-associated SEPT9. (B) The NTE of SEPT9 may inhibit KIF5 motor motility by interfering with the microtubule docking of the KIF5 motor domain, restricting the ATP-triggered swiveling of the neck linker (NL) domain, which underlies ATP hydrolysis and stepping, and/or tethering the stalk domain. (C) Side (left panel) and top-down (right panel) view schematics of structural elements of kinesin-3/KIF1A, which might be in contact with both microtubules and SEPT9. The NTE of SEPT9 may provide additional binding sites for the lysine-rich L12 (left panel) of KIF1A, and interact with loops L11 and L8, helices α4 and α5 and/or the β-sheet β5 of KIF1A, which vary in sequence from KIF5. (D) Side (left panel) and top-down (right) view schematics of dynein-dynactin inhibition. The SEPT9 NTE may occlude the microtubule-binding sites of the CAP-Gly domain of p150^GLUED, and DHC (left panel). Additionally (right panel), it may impede dynein-dynactin from switching protofilaments.