Who pulls the strings in actin cable assembly?

When large structures are built inside of cells, how are their dimensions determined? Are cues received that tell the structure to keep growing, or to slow down, or to stop growing altogether? A recent study published in Developmental Cell by a team led by Molecular and Cell Biology PhD student Melissa Chesarone-Cataldo and Professor of Biology Bruce Goode begins to address these questions by focusing on cytoskeletal structures called yeast actin cables.

Actin cables serve as essential railways for myosin-dependent transport of vesicles, organelles and other cargo, required for yeast cells to grow asymmetrically and produce a daughter cell. Cables are assembled at one end of the mother cell and run the length of the entire cell, but no longer, or else they would hit the back of the cell, buckle and misdirect transport. So how does an actin cable know how long to grow? How are other properties of the cable, such as its thickness and mechanical rigidity determined, and how important are these properties for cable function in vivo?

Actin cables are assembled at the bud neck by the formin protein Bnr1, and rapidly extend into the mother cell at a rate of 0.5-1 µm/s. At this speed, the tip of the actin cable reaches the back end of the cell in about 5-10 seconds. Each cable consists of many shorter overlapping pieces (individual actin filaments) that are stitched or cross-linked together to form a single cable, and cables continuously stream out of the bud neck due to the robust actin assembly activity of Bnr1. Chesarone-Cataldo et al. asked the question, “what mechanism prevents the cables from colliding with the back of the cell and overgrowing?” In doing so, they identified a novel actin cable ‘length sensing’ feedback loop, dependent on the myosin-passenger protein Smy1.

Using live-cell imaging, they showed that Smy1 molecules are transported by myosin from the mother cell to the bud neck, where they pause to interact with the formin Bnr1. Purified Smy1 attenuated Bnr1 activity by slowing down the rate of actin filament elongation. When the SMY1 gene was deleted, cables grew too long, hit the rear of the cell and buckled (see image, right). In addition, the mutant cables abnormally fluctuated in thickness and were kinked, impairing transport of myosin and its cargoes.

The authors propose that a negative feedback loop controls actin cable length. In their model, the cargo (Smy1 in this case) communicates with the machinery that is making the cable (the formin Bnr1), as a means of sensing ‘railway’ length. The longer the railway grows, the more passengers it picks up, and the more transient inhibitory pulses the formin receives. As such, longer cables are selectively attenuated, while shorter cables are allowed to grow rapidly. This negative feedback loop allows yeast cells to tailor actin cable length to the dimensions of the cell and to the needs of its myosin-based transport system.

Current work in the Goode lab is aimed at testing many of the mechanistic predictions of the model above and understanding how Smy1 functions in coordination with other known regulators of Bnr1, all simultaneously present in a cell, to produce actin cables with proper architecture and function. In addition, experiments are underway to find out whether related mechanisms are used to control formins in mammalian cells and to understand the physiological consequences of disrupting those mechanisms.

Chesarone-Cataldo M, Guérin C, Yu JH, Wedlich-Söldner R, Blanchoin L, Goode BL. The Myosin passenger protein Smy1 controls actin cable structure and dynamics by acting as a formin damper. Dev Cell. 2011 Aug 16;21(2):217-30.

Actin assembly and colorectal cancer

A new paper in J. Cell Biology from Bruce Goode’s lab by postdoc Kyoko Okada and co-workers finds that Adenomatous polyposis coli protein nucleates actin assembly. This may suggest a potential role of APC in suppressing tumors through its effects on actin assembly.

BIOL 99 AND NEUR 99 Senior Honors Talks

Senior honors presentations and defenses for Biology and Neuroscience are this week and next Monday.

Name      Faculty Sponsor & Committee  Time & Location of Talk

Biol 99

Alicia Bach Dagmar Ringe, Neil Simister, Liz Hedstrom May 10   3 pm    Bassine 251
Kristin Little Bruce Goode, Joan Press, Satoshi Yoshida May 6    10 am    Bassine 251
Spencer Rittner KC Hayes, Carolyn Cohen, Larry Wangh May 6    3 pm      Bassine 251
Danielle Saly Michael Rosbash, Mike Marr, Nelson Lau May 10   11 am   Bassine 251
Sue Yen Tay Jim Haber, Sue Lovett, Joan Press  May 7    11am     Bassine 251
Alan Tso Daniela Nicastro, Liz Hedstrom, Greg Petsko May 10   2 pm    Bassine 251
Hannah Worchel Jim Morris, Ruibao Ren, Paul Garrity   May 6    2 pm     Bassine 251

Neur 99
Sarah Pease Sue Paradis, Gina Turrigiano, Paul Miller  May 10   11 am   Volen 201
Solon Schur John Lisman, Eve Marder, Paul Miller May 6    10 am   Volen 201
Alexander Trott Leslie Griffith, Piali Sengupa, Melissa Kosinski-Collins May 6    11 am   Volen 201
Dylan Wolman Sue Paradis, Sacha Nelson, Piali Sengupta May 10   1 pm    Volen 201

Faculty research mentor (emphasized) is chair of the committee.

How actin networks assemble in cells

A new review article in Current Opinion in Cell Biology by Molecular and Cell Biology grad student Melissa Chesarone and Biology’s Professor Bruce Goode focuses on a group of remarkable protein machines that rapidly assemble actin polymers in cells. These factors are essential for cell division, cell movement, and cell shape determination in virtually all organisms. Their catalytic mechanisms involve intricate fast-moving parts, which enables them to construct entire actin networks in a matter of seconds.

Actin "pointers" for EM labeling

Single particle electron microscopy reconstruction can be a powerful tool for determining the structure of large protein complexes. One limitation of the technique is the difficulty in coming up with specific labels for the protein that can be visualized with EM. In a new paper in RNA, postdoc Beth Stroupe and coworkers show that the use of the actin-nucleating protein Spire as a cloneable tag allows them to nucleate actin filaments that then “point” to the location of the tag in the complex seen in EM, and applied the technique to their studies of the C complex spliceosome.

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