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A Unique Case of Collaboration in the Microscopic World Between Two Stanford Labs

The exchange of ideas and information among scientists is nothing new. Now, in fitting parallel, a collaboration between two Stanford labs has revealed a unique case of collaboration in the microscopic world. The results are published in the January 25 edition of the journal Cell.

The GTPase proteins are a large family of enzymes involved in a host of activities inside cells. Using X-ray diffraction at the Stanford Synchrotron Radiation Laboratory (SSRL) beamlines 11-1 and 7-1, researchers from the labs of Stanford professors Axel Brunger and Suzanne Pfeffer have found an unexpected instance of two different GTPase proteins working together. Brunger, whose primary focus is the mechanics of neurotransmission, and Pfeffer, who studies how receptors are moved around in mammalian cells, joined forces to look at how two different GTPase proteins function at the membrane surface of a cellular organelle called the Golgi complex.

Proteins are transported to specific sites within cells enclosed in packets called transport vesicles, which are moved along a specialized network of tracks called microtubules. The Golgi complex is a central sorting station in cells, and is at the center of the cell's secretion machinery. But exactly how vesicles carrying incoming proteins recognize the Golgi as their correct targets is poorly understood.

"Cellular trafficking is important for just about everything in the cell," said Alondra Schweizer Burguete, who led the study. "We know the least about how vesicles recognize their targets. Of all the vesicle transport steps this is least defined." Together with Timothy Fenn, Burguete, who previously worked in the Pfeffer lab and now conducts postdoctoral research in the Brunger lab, bridges the strengths of the two labs on the current research.

Burguete and her colleagues determined the structure of part of a tethering protein in complex with a pair of GTPase proteins called Rab6. They showed that anchoring the tethering protein to the Golgi requires cooperation between two families of GTPases: the Rab family and the Arl family. Only together could the two GTPases provide stable binding of the tether to the Golgi complex.

Having determined the structure of Rab6 in complex with the tether, the team then combined that information with the previously studied Arl1 structure to create a model of the system. The model explains how the Arl1 and Rab6 GTPase pairs may cooperate with each other in directing incoming vesicles to the Golgi membrane prior to fusion. This represents a first-ever instance of such a cooperative relationship among different members of these GTPase families.

"I think what's exciting about our finding is the sort of crosstalk between different G proteins, in this case by having a common binding partner," said Brunger. "This is one of the first examples where we have shown that they interact cooperatively."

"Stanford is a wonderful place because the environment fosters interactions between research groups, departments, with SLAC and across the University," said Pfeffer. "It's a very collaborative and interactive research environment."

Note for Neurotransmission
Neurotransmission, also called synaptic transmission, is an electrical movement with-in the synapses caused by a propagation of nerve impulses. As each nerve cell receives the neurotransmitter, exchanging them from the presynaptic neuron, or terminal button, to the postsynaptic neuron, or dendrite, of the second neuron, it sends it back out to several neurons, and they do the same, thus creating a wave of energy until the pulse has made its way across an organ or specific area of neurons. .

All experiences, such as thoughts and feelings, and all actions, are the results of neurons generating nerve impulses. Without nerve impulses an organism is clinically dead, so they are essential for the organism's existence.
Neurons form networks through which nerve impulses travel. Each neuron receives as many as 15000 connections from other neurons. Neurons don't touch each other, but they have contact points, that are called synapses. A neuron transports its information by way of a nerve impulse. When a nerve impulse arrives at the synapse, it releases neurotransmitters, which influence another neuron, either in an inhibitory way or in an excitatory way. This next neuron is connected to many more neurons, and if the total of excitatory influences is more than the inhibitory influences, it will also "fire", that is, it will create a new action potential at its axon hillock, in this way passing on the information to yet another next neuron, or resulting in an experience or an action.

Note for Rab (G-protein)
The Rab family of proteins is a member of the Ras superfamily of monomeric G proteins. Approximately 70 types of Rabs have now been identified in humans. Rab GTPases regulate many steps of membrane traffic, including vesicle formation, vesicle movement along actin and tubulin networks, and membrane fusion. These processes make up the route through which cell surface proteins are trafficked from the Golgi to the plasma membrane and are recycled. Surface protein recycling returns proteins to the surface whose function involves carrying another protein or substance inside the cell, such as the transferrin receptor, or serves as a means of regulating the number of a certain type of protein molecules on the surface.

Rab proteins are peripheral membrane proteins, meaning they are anchored to a membrane via a lipid group covalently linked to an amino acid. Specifically, Rabs are anchored via prenyl groups on two cysteines in the C-terminus. Rab escort proteins (REPs) deliver newly-synthesized and prenylated Rab to its destination membrane by binding the hydrophobic, insoluble prenyl groups and carrying Rab through the cytoplasm. The lipid prenyl groups can then insert into the membrane, anchoring Rab at the cytoplasmic face of a vesicle or the plasma membrane. Because Rab proteins are anchored to the membrane through a flexible C-terminal region, they can be thought of as a 'balloon on a string'.

Note for GTPases
GTPases (singular GTPase) are a large family of hydrolase enzymes that can bind and hydrolyze GTP. The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.
They help GTP binding proteins hydrolyse GTP and be converted to their ground state.

GTPases play an important role in:
Signal transduction at the intracellular domain of transmembrane receptors, including recognition of taste, smell and light.
Protein biosynthesis (aka translation) at the ribosome.
Control and differentiation during cell division.
Translocation of proteins through membranes.
Transport of vesicles within the cell. (GTPases control assembly of vesicle coats).

About Stanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Laboratory, a division of Stanford Linear Accelerator Center, is operated by Stanford University for the Department of Energy. SSRL is a National User Facility which provides synchrotron radiation, a name given to x-rays or light produced by electrons circulating in a storage ring (SPEAR) at nearly the speed of light. These extremely bright x-rays can be used to investigate various forms of matter ranging from objects of atomic and molecular size to man-made materials with unusual properties. The obtained information and knowledge is of great value to society, with impact in areas such as the environment, future technologies, health, and education.

The SSRL provides experimental facilities to some 2,000 academic and industrial scientists working in such varied fields as drug design, environmental cleanup, electronics, and x-ray imaging.

In figure, (From left) Axel Brunger, Suzanne Pfeffer, Alondra Schweizer Burguete and Timothy Fenn solved the structure of the Rab6 protein (pictured), important for transport of proteins within cells.