Featured System - October 2009
Short description: Our cells are filled with a cytoskeleton that provides an infrastructure for support, transport, and locomotion.
Our cells are filled with a cytoskeleton that provides an infrastructure for support, transport, and locomotion. It is comprised of a network of filaments--actin, intermediate filaments and microtubules--connected by a bewildering variety of receptors, connectors, and motors. Unlike our larger skeleton of bones, however, the cytoskeleton is highly dynamic and is constantly remodeled as the needs of the cell change.
Rearrangement of the cytoskeleton is controlled in part by the Rho family of GTPases, such as Rac1, which in turn are controlled by guanine nucleotide exchange factors such as Vav1. Vav1 specializes in control of the actin cytoskeleton in blood cells. These types of rearrangements are critical, for example, for the formation of the immunological synapse that strengthens the interaction of T-cells with their targets, and for changing shape of macrophages as they engulf their targets. Vav1 may also play a role in the cytoskeletal changes that occur during cancerous transformation of a cell, which make them more mobile and invasive than normal cells.
The Rho GTPases are one of the major players in the cell's network of communication. They are small proteins that act as an on-off switch. They have two states: if they have a GTP bound, they are "on" and they activate their protein targets. The GTP then slowly hydrolyzes, turning the signal off. As you might imagine, however, cells don't rely on this slow, uncontrolled breakdown of GTP. Instead, they also build proteins that enhance the hydrolysis, breaking down the GTP quickly and exactly when needed. They also build proteins like Vav1 that extract the spent GDP and replace it with GTP, reactivating the Rho GTPase.
Researchers at the ATCG3D and their collaborators have taken a close look at how guanine nucleotide exchange factors perform their activating task. Vav1, shown here in blue and green, is a large multidomain protein that exchanges guanine nucleotides from many different Rho GTPases, such as Rac1, shown here in red. This is unusual, because in most cases, there are specific pairings of a particular guanine nucleotide exchange factor with a particular Rho GTPase. The structural basis of the unusual promiscuity of Vav1 was revealed in the structure, which is available in PDB entry 3bji.
The fragment of Vav1 seen in the structure includes three domains (several others, which are involved with interaction with the cytoskeleton and with other signaling molecules, are not included in the structure). The largest domain, termed the dbl homology (DH) domain, interacts with Rho GTPases, changing the conformation of two "switch" loops that promote the exchange of the nucleotides. The second domain, termed the pleckstrin homology (PH) domain, is involved with interaction with lipids. The third, cysteine-rich domain (CRD) is the key. It loops back and interacts with a long alpha helix in the DH domain. This stabilizes the conformation of the DH domain and makes it perfect for interacting with the switch loops of Rho GTPases. To take a closer look at this interaction, click on the image below for an interactive Jmol view.
The JSmol tab below displays an interactive JSmol.
Secretagogin is composed of six calcium-binding EF-hand motifs (colored in rainbow colors here) connected by short linker regions (shown in gray). Notice that they are arranged in pairs to form three globular domains. You can use the buttons below to display the sidechains that are predicted to form the calcium-binding site in each one.
Chrencik, J. E., Brooun, A., Zhang, H., Mathews, I. I., Hura, G. L., Foster, S. A., Perry, J. J. P., Streiff, M., Ramage, P., Widmer, H., Bokoch, G. M., Tainer, J. A., Weckbecker, G. and Kuhn, P. (2008) Structural basis of guanine nucleotide exchange mediated by the T-cell essential Vav1. J. Mol. Biol. 380, 828-843.
Hornsteine, I., Alcover, A. and Katzav, S. (2004) Vav proteins, masters of the world of cytoskeleton organization. Cell. Signalling 16, 1-11.
Rossman, K. I., Der, C. J. and Sondek, J. (2005) GEF means go: turning on Rho GTPases with guanine nucleotide-exchange factors. Nat. Rev. Mol. Cell Biol. 6, 167-180.