The spectraplakin Short stop is an actin-microtubule cross-linker that contributes to organization of the microtubule network.

The dynamics of actin and microtubules are coordinated in a variety of cellular and morphogenetic processes; however, little is known about the molecules mediating this cytoskeletal cross-talk. We are studying Short stop (Shot), the sole Drosophila spectraplakin, as a model actin-microtubule cross-linking protein. Spectraplakins are an ancient family of giant cytoskeletal proteins that are essential for a diverse set of cellular functions; yet, we know little about the dynamics of spectraplakins and how they bridge actin filaments and microtubules. In this study we describe the intracellular dynamics of Shot and a structure-function analysis of its role as a cytoskeletal cross-linker. We find that Shot interacts with microtubules using two different mechanisms. In the cell interior, Shot binds growing plus ends through an interaction with EB1. In the cell periphery, Shot associates with the microtubule lattice via its GAS2 domain, and this pool of Shot is actively engaged as a cross-linker via its NH(2)-terminal actin-binding calponin homology domains. This cross-linking maintains microtubule organization by resisting forces that produce lateral microtubule movements in the cytoplasm. Our results provide the first description of the dynamics of these important proteins and provide key insight about how they function during cytoskeletal cross-talk.

Kif5b is an essential forward trafficking motor for the Kv1.5 cardiac potassium channel.

We have investigated the role of the kinesin I isoform Kif5b in the trafficking of a cardiac voltage-gated potassium channel, Kv1.5. In Kv1.5-expressing HEK293 cells and H9c2 cardiomyoblasts, current densities were increased from control levels of 389 +/- 50.0 and 317 +/- 50.3 pA pF(1), respectively, to 614 +/- 74.3 and 580 +/- 90.9 pA pF(1) in cells overexpressing the Kif5b motor. Overexpression of the Kif5b motor increased Kv1.5 expression additively with several manipulations that reduce channel internalization, suggesting that it is involved in the delivery of the channel to the cell surface. In contrast, expression of a Kif5b dominant negative (Kif5bDN) construct increased Kv1.5 expression non-additively with these manipulations. Thus, the dominant negative acts by indirectly inhibiting endocytosis. The increase in Kv1.5 currents induced by wild-type Kif5b was dependent on Golgi function; a 6 h treatment with Brefeldin A reduced Kv1.5 currents to control levels in Kif5b-overexpressing cells but had little effect on the increase associated with Kif5bDN expression. Finally, expression of the Kif5bDN prior to induction of Kv1.5 in a tetracycline inducible system blocked surface expression of the channel in both HEK293 cells and H9c2 cardiomyoblasts. Thus, Kif5b is essential to anterograde trafficking of a cardiac voltage-gated potassium channel.

Internalized Kv1.5 traffics via Rab-dependent pathways.

Little is known about the postinternalization trafficking of surface-expressed voltage-gated potassium channels. Here, for the first time, we investigate into which of four major trafficking pathways a voltage-gated potassium channel is targeted after internalization. In both a cardiac myoblast cell line and in HEK293 cells, channels were found to internalize and to recycle quickly. Upon internalization, Kv1.5 rapidly associated with Rab5-and Rab4-positive endosomes, suggesting that the channel is internalized via a Rab5-dependent pathway and rapidly targeted for recycling to the plasma membrane. Nevertheless, as indicated by colocalization with Rab7, a fraction of the channels are targeted for degradation. Recycling through perinuclear endosomes is limited; colocalization with Rab11 was evident only after 24 h postsurface labelling. Expression of dominant negative (DN) Rab constructs significantly increased Kv1.5 functional expression. In the myoblast line, Rab5DN increased Kv1.5 current densities to 1305 +/- 213 pA pF(-1) from control 675 +/- 81.6 pA pF(-1). Rab4DN similarly increased Kv1.5 currents to 1382 +/- 155 pA pF(-1) from the control 522 +/- 82.7 pA pF(-1) at +80 mV. Expression of the Rab7DN increased Kv1.5 currents 2.5-fold in HEK293 cells but had no significant effect in H9c2 myoblasts, and, unlike the other Rab GTPases tested, over-expression of wild-type Rab7 decreased Kv1.5 currents in the myoblast line. Densities fell to 573 +/- 96.3 pA pF(-1) from the control 869 +/- 135.5 pA pF(-1). The Rab11DN was slow to affect Kv1.5 currents but had comparable effects to other dominant negative constructs after 48 h. With the exception of Rab11DN and nocodazole, the effects of interference with microtubule-dependent trafficking by nocodazole or p50 overexpression were not additive with the Rab dominant negatives. The Rab GTPases thus constitute dynamic targets by which cells may modulate Kv1.5 functional expression.

The F-actin-microtubule crosslinker Shot is a platform for Krasavietz-mediated translational regulation of midline axon repulsion.

Axon extension and guidance require a coordinated assembly of F-actin and microtubules as well as regulated translation. The molecular basis of how the translation of mRNAs encoding guidance proteins could be closely tied to the pace of cytoskeletal assembly is poorly understood. Previous studies have shown that the F-actin-microtubule crosslinker Short stop (Shot) is required for motor and sensory axon extension in the Drosophila embryo. Here, we provide biochemical and genetic evidence that Shot functions with a novel translation inhibitor, Krasavietz (Kra, Exba), to steer longitudinally directed CNS axons away from the midline. Kra binds directly to the C-terminus of Shot, and this interaction is required for the activity of Shot to support midline axon repulsion. shot and kra mutations lead to weak robo-like phenotypes, and synergistically affect midline avoidance of CNS axons. We also show that shot and kra dominantly enhance the frequency of midline crossovers in embryos heterozygous for slit or robo, and that in kra mutant embryos, some Robo-positive axons ectopically cross the midline that normally expresses the repellent Slit. Finally, we demonstrate that Kra also interacts with the translation initiation factor eIF2beta and inhibits translation in vitro. Together, these data suggest that Kra-mediated translational regulation plays important roles in midline axon repulsion and that Shot functions as a direct physical link between translational regulation and cytoskeleton reorganization.

Distinct sites in E-cadherin regulate different steps in Drosophila tracheal tube fusion.

We have investigated how E-cadherin controls the elaboration of adherens junction associated cytoskeletal structures crucial for assembling tubular networks. During Drosophila development, tracheal branches are joined at branch tips through lumens that traverse doughnut-shaped fusion cells. Fusion cells form E-cadherin contacts associated with a track that contains F-actin, microtubules, and Shot, a plakin that binds F-actin and microtubules. Live imaging reveals that fusion occurs as the fusion cell apical surfaces meet after invaginating along the track. Initial track assembly requires E-cadherin binding to beta-catenin. Surprisingly, E-cadherin also controls track maturation via a juxtamembrane site in the cytoplasmic domain. Fusion cells expressing an E-cadherin mutant in this site form incomplete tracks that contain F-actin and Shot, but lack microtubules. These results indicate that E-cadherin controls track initiation and maturation using distinct, evolutionarily conserved signals to F-actin and microtubules, and employs Shot to promote adherens junction-associated cytoskeletal assembly.

Chemotactically directed redistribution of alpha-actinin precedes morphological polarization and reversal of polarity in human polymorphonuclear leucocytes (PMNs).

We have determined the temporal and spatial relationship between cell polarization and alpha-actinin localization by analysing the redistribution of alpha-actinin and F-actin in spherical PMNs developing polarity and in polarized cells reversing polarity following localized stimulation with chemotactic peptide using micropipettes. Initially spherical PMNs develop a one-sided accumulation of alpha-actinin before lamellipodia enriched in alpha-actinin are formed. In polarized cells, alpha-actinin is concentrated at the leading front. When polarity is reversed, alpha-actinin redistribution to the uropod precedes reversal of morphological polarity and formation of new lamellipodia at the uropod. Later, lamellipodia enriched in F-actin and alpha-actinin develop at the former uropod to form a new front. The data document that redistribution of alpha-actinin is a very early event in the development of polarity, which precedes formation of lamellipodia.

Localised depletion of polymerised actin at the front of Walker carcinosarcoma cells increases the speed of locomotion.

Spontaneously migrating Walker carcinosarcoma cells usually form lamellipodia at the front. Combined treatment with 10(-5)M colchicine and 10(-7)M latrunculin A produces large defects in the cortical F-actin layer at the leading front and suppresses lamellipodia. However, the cortical actin layer at the rear is intact and shows myosin IIA accumulation. These cells, showing no or little detectable cortical F-actin at the front and no morphologically recognisable protrusions, migrate faster than control cells with lamellipodia and an intact cortical actin layer. This documents that the cortical actin layer or actin-powered force generation at the front is redundant for locomotion. Colchicine and latrunculin A have synergistic effects in compromising the cortical layer at the front and in increasing the speed of locomotion, but antagonistic effects on the relative amount of F-actin per cell. Colchicine but not latrunculin A, can increase the proportion of polarised and locomoting cells under appropriate conditions. Locomotion and polarity of cells treated with latrunculin A and colchicine is inhibited at latrunculin A concentrations >10(-7)M, by the myosin inhibitor BDM or the ROCK inhibitor Y-27632. Colchicine and Y-27632 have antagonistic effects on polarity and the speed of locomoting cells. The data show that locomotion of metazoan cells, which normally form lamellipodia, can be driven by actomyosin contraction behind the front (cell body, uropod). They are best compatible with a cortical contraction/frontal expansion model, but they are not compatible with models implying that actin polymerisation or actomyosin contraction at the front drive locomotion of the cells studied.