January 15, 2014

The mitotic spindle seems to get all the fun of a microtubule-dynein party, but do not fret. A recent paper describes some cool interactions of microtubules with dynein at the cell’s cortex.

The molecular motor dynein walks along microtubules, and this movement can do great things by moving the microtubules themselves or moving material along the microtubule. Recent work found that dynein at the cell’s cortex may influence cell motility using an actin-independent mechanism that pushes microtubules along the cortex. In an even more recent paper in the journal Molecular Biology of the Cell, this same research group shows these cortical dynein-microtubule interactions directly. Using TIRF microscopy, Mazel and colleagues found speckles of cortical dynein complexes associated with microtubules. These microtubules can move, bend, and even rotate around these speckles. The images above show the difference between wide-field microscopy (left) and TIRFM (right) when imaging microtubules at the cortex. In the bottom panel, a short microtubule can be seen moving directionally.


Tomáš Mazel, Anja Biesemann, Magda Krejczy, Janos Nowald, Olga Müller, & Leif Dehmelt (2014). Direct observation of microtubule pushing by cortical dynein in living cells Molecular Biology of the Cell, 25 (1) DOI: 10.1091/mbc.E13-07-0376

April 16, 2013

I’m as type-A as a person can get, with my organized desk, to-do lists, and inability to roll with it (I sound dreadful, right?).  One thing that will never change is the calm I feel when I read the word “systematic” in a paper’s title or abstract.  Anything done systematically soothes me like a heartbeat soothes a newborn.  Today’s image is from a paper that uses a—you guessed it—systematic approach to understanding the roles of dynein and its many regulators in mitosis.

Dynein is a large microtubule motor complex that is important in countless cellular processes, most notably mitosis.  Like most proteins, dynein relies on numerous adaptor proteins and the dynactin complex to help localize the motor and/or activate it.  A recent paper uses siRNA screening to systematically test the roles of dynein subunits, adaptor proteins, and dynactin subunits to build a more complete picture of the roles of each protein in mitosis.  Raaijmakers and colleagues show that while some regulators are required for activation but not localization of dynein, others are required mainly for dynein localization.  Dynactin, for example, is not necessary for spindle organization, but rather serves as a dynein recruitment factor at the nuclear envelope and kinetochores.  In the images above, the mitotic spindle in a control cell is very focused at spindle poles (second row, green in merged).  When the dynein heavy chain subunit is depleted from cells (all other columns), spindles show a range of spindle pole focusing defects, including spindles lacking attachment to their poles.  Spindle microtubules are top row, red in merged; chromosomes are third row, blue in merged.

Raaijmakers, J., Tanenbaum, M., & Medema, R. (2013). Systematic dissection of dynein regulators in mitosis originally published in the Journal of Cell Biology, 201 (2), 201-215 DOI: 10.1083/jcb.201208098

October 4, 2012

What happens when you bake a cake without baking powder? That’s right…you get a flat, dense cake-shaped hockey puck. What about too much baking powder? Kablooey…the cake rises quickly then deflates. Approaching questions in cell biology this same way leads to amazing discoveries about cells, minus the cake. Today’s image is from a paper describing the roles of different dynactin subunits.

Dynactin is a multi-protein complex that is essential for the activity and binding of dynein, the microtubule motor. It has been suggested that dynactin is the key to dynein’s ability to bind countless different cellular cargoes throughout every stage of the cell cycle. Of the 11 subunits in dynein, 4 proteins (Arp11, p62, p27, and p25) make up the “pointed end complex,” a domain believed to be important in cargo specificity. A recent paper shows results on the importance and interactions of these pointed end complex proteins. By depleting levels—or overexpressing—these proteins in tissue culture cells, Yeh and colleagues found that p62 and Arp11 pair up and affect dynactin binding to the nuclear envelope prior to mitosis, while p27 and p25 pair up to regulate membrane binding (early and recycling endosomes). Yeh and colleagues also found that Arp11 and p62 are necessary for dynactin stability. In the images above, mitotic spindles lacking different subunits of the pointed end complex show various defects. The spindles (red, microtubules) of cells lacking Arp11 and p62 are multipolar, as compared to wild-type cells (top row). Spindles of cells lacking p27, however, appeared normal.


Yeh TY, Quintyne NJ, Scipioni BR, Eckley DM, & Schroer TA (2012). Dynactin's pointed-end complex is a cargo-targeting module. Molecular biology of the cell, 23 (19), 3827-37 PMID: 22918948

February 16, 2012

Dynein is a microtubule motor that resides at the cortex of a cell and can position an entire mitotic spindle. To visualize this, it would help if you were aware of one my killer dance moves…I stand in one spot and reel in a friend with my invisible lasso to dance. And, that friend may or may not have a look of total embarrassment (pity?) on his or her face. While dynein will never beat me in a dance-off, it is a pretty spectacular motor protein. Check out today’s image from a paper describing exactly how dynein can generate pulling forces.

Proper positioning of the mitotic spindle is important for cell division, especially when a cell has to divide asymmetrically to result in two cells of different sizes. Dynein is a microtubule motor that resides at the cortex of a dividing cell and can reel in and position an entire spindle. Cortical dynein functions this way in contexts outside of mitosis too—during migration, for example, dynein may help position the microtubule-nucleating centrosome correctly for trafficking of membrane vesicles. Recently, a group of cell biologists looked at exactly how dynein interacts with microtubules to generate a pulling force, and did so by taking dynein and microtubule asters out of cells and into chambers with microfabricated barriers. In this paper, Laan and colleagues looked at how dynein, attached to the fabricated barriers, interacted with microtubules. By capturing microtubules head-on, dynein regulated microtubule dynamics and length. When the microtubule ends were shrinking, dynein generated a pulling force strong enough to center the microtubule aster in the chamber. Images above show microtubule asters in the microchambers with barriers either coated with dynein or not. Without dynein at the barrier, microtubules continued to grow after reaching the barrier then buckled. With dynein-coated barriers to interact with, microtubules were captured by dynein and stopped growing, mostly remaining straight.

Laan, L., Pavin, N., Husson, J., Romet-Lemonne, G., van Duijn, M., López, M., Vale, R., Jülicher, F., Reck-Peterson, S., & Dogterom, M. (2012). Cortical Dynein Controls Microtubule Dynamics to Generate Pulling Forces that Position Microtubule Asters Cell, 148 (3), 502-514 DOI: 10.1016/j.cell.2012.01.007
Copyright ©2012 Elsevier Ltd. All rights reserved.

November 3, 2011

I love a hidden picture task. I love looking at a picture of a crowded street scene and identifying the nerd in the red and white sweater (side note: would Waldo’s fashion choices put him in the hipster category these days?). In cell biology, a researcher has to sort through the crowded scene in a cell to find what he or she is looking for. Today’s image is from a paper describing the function of microtubule motors, a difficult job given the complexity and interdependence of the motors and their regulators.

Microtubule motors called dynein and kinesin move all sorts of material around the cell. The motor binds to its cargo, a membrane vesicle for example, and “walks” it along a microtubule until it reaches its destination, such as an endosome or lysosome in this example. With multiple motors in any given cell type and a slew of regulators for each, the understanding of an individual motor’s contribution is unclear. A recent paper helps to sort through this complexity. In this paper, Yi and colleagues used acute inhibition of dynein and its regulators, followed by precise tracking of particles in a cell. Following the inhibition of dynein, multiple cargoes rapidly disperse around the cell, suggesting a sharp drop in minus-end directed transport along microtubules. In the images above, the top row shows cells at the time of the dynein inhibition, while bottom row shows several minutes later. Lysosomes/late endosomes, early endosomes, Golgi, and injected adenovirus (left to right) all dispersed towards the cell periphery following dynein inhibition. Interestingly, Yi and colleagues also saw a gradual decrease in transport in the other direction (plus-end directed) following dynein inhibition, suggesting a possible global effect on transport.

Yi, J., Ori-McKenney, K., McKenney, R., Vershinin, M., Gross, S., & Vallee, R. (2011). High-resolution imaging reveals indirect coordination of opposite motors and a role for LIS1 in high-load axonal transport originally published in The Journal of Cell Biology, 195 (2), 193-201 DOI: 10.1083/jcb.201104076

October 3, 2011

Behind every great mobile organelle is an equally awesome motor protein. The motor proteins dynein and kinesin move cargo along microtubules, and play crucial roles in countless cellular processes. A recent paper shows how these two motors cooperate.

The fungus Ustilago maydis grows into long hyphal cells in laboratory culture. Their use in cell biology is powerful, as their length and motor transport is reminiscent of human neurons. These long cells grow from the cell tip and have similarly oriented microtubules at either end of the cell. In the middle of the cell, microtubules overlap with opposite polarity. The polarity of these microtubules is important – dynein motors walk to one end of microtubules (the “minus” end), while most kinesin motors walk to the other (the “plus” end). A recent paper looked at how these two motors cooperate with each other in the motility of early endosomes in U. maydis cells. Schuster and colleagues found that while dynein is important for short-range motility, kinesin is important for long-range transport through the antipolar microtubule array in the center of the cell. Top image above shows the elongated hyphal cell with the nucleus in red. Bottom image shows the growth of microtubules by showing two consecutive time-points of EB1 (red then green), which is a protein found on the tips of growing microtubules. The two different insets show the antipolar growth of microtubules at the center (left inset), compared with the growth of similarly-oriented microtubules near the cell tip (right inset).

Schuster, M., Kilaru, S., Fink, G., Collemare, J., Roger, Y., & Steinberg, G. (2011). Kinesin-3 and dynein cooperate in long-range retrograde endosome motility along a nonuniform microtubule array Molecular Biology of the Cell, 22 (19), 3645-3657 DOI: 10.1091/mbc.E11-03-0217