JCB biowrites

ASCB 2011: The ubiquity of ubiquitin

Co-chairs Claudio Joazeiro (The Scripps Research Institute) and Frauke Melchior (University of Heidelberg) put together a minisymposium on the third day of ASCB 2011 that nicely highlighted the diversity of cellular functions performed by ubiquitin and ubiquitin-like proteins.

Ubiquitin is best known for its role in protein degradation when E3 ubiquitin ligases tag proteins with ubiquitin in order to target them to the proteasome. But, as Joazeiro described in his opening talk, individual E3 ligases can take part in very specific degradation pathways, a fact exemplified by Listerin, an E3 ligase whose mutation in mice causes motor neuron degeneration. Joazeiro showed that the yeast homolog of Listerin, Ltn1, is involved in a type of protein degradation linked to an mRNA quality control pathway called non-stop mRNA decay. This pathway rids cells of mRNAs that lack a proper stop codon, but an abnormal polypeptide is always produced before the mRNA can be eliminated. Joazeiro showed that Ltn1 associates with ribosomes and targets for destruction the proteins produced from non-stop mRNAs.

Probably the second-best known role for ubiquitination is in cell signaling. Fumiyo Ikeda (IMBA, Vienna) presented data from her recent paper describing how SHARPIN - a component of the Linear Ubiquitin Chain Assembly Complex (LUBAC) - regulates two signaling pathways. SHARPIN promotes NFkappaB signaling by mediating the ubiquitination of and NFkappaB regulator called NEMO, and it also inhibits cell death by restricting apoptotic signaling through the death domain-containing protein FADD. SHARPIN-deficient mice therefore suffer from increased apoptosis and inflammation.

The following two talks dealt with some less-appreciated roles of ubiquitination. Corinne Albiges-Rizo (Institut Albert Bonniot, Grenoble) discussed how ubiquitination of an integrin-binding protein helps regulate cell adhesion and migration. And Yanzhuang Wang (University of Michigan) described how the E3 ubiquitin ligase HACE1 is required for the reassembly of the Golgi apparatus after mitosis. (Interestingly, HACE1 is a tumor suppressor. Is there a connection between a failure to reassemble the Golgi and tumorigenesis?)

The final two talks examined the cellular functions of ubiquitin-like molecules. Jay Debnath (UC San Francisco) spoke about Atg12, a ubiquitin-like modifier required for expanding autophagosomal membranes during macroautophagy. Debnath and colleagues found that Atg12 can be conjugated to Atg3, a protein involved in the conjugation of another ubiquitin-like protein, LC3, that is also required for autophagy. The Atg12-Atg3 conjugate isn't required for starvation-induced autophagy, however. Instead, preventing the formation of Atg12-Atg3 conjugates blocks the related pathway of mitophagy (in which mitochondria are degraded) and also inhibits mitochondrial fusion, resulting in cells with increased numbers of highly-fragmented mitochondria. Debnath thinks that Atg12 may be conjugated to several other proteins and have important functions in a number of other cellular pathways as well.

And session co-chair Frauke Melchior finished off the session with a presentation on how oxidative stress regulates the small, ubiquitin-like modifier SUMO. Sumoylation is a reversible post-translational modification that affects the function of many different proteins. Reactive oxygen species cause a reduction in the sumoylation of many target proteins. In 2006, Melchior published that ROS inhibit sumoylation by promoting the formation of a disulphide bond between catalytic cysteine residues in two enzymes required for sumoylation, blocking the activity of both proteins. Melchior now described some follow up experiments to this initial report, looking at the potential functional consequences to cells if oxidative stress fails to shut down sumoylation.

 

 

ASCB 2011: Simulating the spindle

Today at ASCB, Francois Nedelec (EMBL) gave a great talk in the "Self-Organization of Cellular Structures" symposium about using the Cytosim program to model cytoskeletal dynamics. His presentation drew heavily on his JCB paper modeling the Xenopus meiotic spindle, which you can learn about in this biosights video we made to accompany the study...

 

ASCB 2011: A cascade of Rabs and Arfs

On the opening day of the annual ASCB meeting in Denver, CO, I decided to check out the 'Special interest subgroup' on Rabs and Arfs - two families of small GTPases that regulate a variety of membrane trafficking events in eukaryotic cells.

The session - organized by Elizabeth Sztul (University of Alabama) and Angela Wandinger-Ness (University of New Mexico) - covered a variety of the different functions performed by these GTPases and the many proteins that regulate them. Cathy Jackson (CNRS, France) discussed how a guanine nucleotide exchange factor (GEF) that activates the Arf1 GTPase helps deliver key proteins to fat-storing lipid droplets, while James Goldenring (Vanderbilt University) described how the regulation of endocytic recycling pathways in polarized epithelial cells by Rab25 helps to suppress colon cancer. Wandinger-Ness explained how mutations in Rab7 cause Charcot-Marie-Tooth disease Type 2B, and discussed how Rabs might be targeted therapeutically by small molecules.

Nava Segev (University of Illinois) discussed the regulation of Rab proteins (known as Ypts in budding yeast). In addition to GEFs and other factors that regulate Rab localization and activation into the GTP-bound state, Segev explained that the choice of effector protein that interacts with the active GTPase is also a critical control point that allows Rabs to function in multiple different pathways. For example, the yeast Rabs Ypt31 and Ypt32 function in both Golgi-to-plasma membrane and endosome-to-Golgi trafficking, using different effectors for each of the two pathways. 

Roger Goody (MPI of Molecular Physiology, Germany) stressed the importance of GEFs in determining when and where Rabs are activated, and described how normal Rab function can be subverted by pathogens such as Legionella. Once Legionella have been internalized into a vacuole within a host cell, they secrete a number of factors that induce the recruitment of ER-derived vesicles that eventually convert the Legionella-containing vacuole into something that resembles the ER. One of these secreted factors is a protein called DrrA, which acts as a GEF that activates and recruits the host's Rab1 molecules in order to promote ER-vesicle transport to the Legionella-containing vacuole. Moreover, DrrA catalyzes the addition of an AMP-molecule onto Rab1, blocking its association with host cell regulatory and effector proteins. But AMPylated Rab1 can still interact with Legionella LidA (a 'super effector' that interacts with multiple host cell Rabs), suggesting that DrrA and LidA work together to recruit early secretory vesicles to Legionella-containing vacuoles. For a broader view of how pathogens subvert host cell membrane trafficking pathways, read this recent review from JCB. Meanwhile, Rick Kahn (Emory University) discussed the potential contribution of vesicular cargo to determining exactly where and when GEFs are prompted to activate GTPases to stimulate vesicle transport.

One recurring theme in the session was that of GTPase cascades, in which a series of Arfs and Rabs either promote or inhibit each other's activation. Arnaud Echard (Institut Pasteur, France) revealed how two GTPases that antagonize each other to ensure that cells complete cytokinesis; co-chair Elizabeth Sztul explained how one GTPase activates another to recruit different vesicle coats to the Golgi; and Sean Munro (MRC-LMB, UK) described a chain of three GTPases that act consecutively to recruit proteins to Golgi membranes.

All in all, there was plenty to ponder on from this session. With Arfs and Rabs regulating so many critical transport processes in cells, it's no wonder that their regulation is so complex. It seems that many questions remain about how cells decide exactly when and where to switch these GTPases on.