Cell Biochemistry

Edited by: Thomas W. Durson M. Søgaard, K. Tani, R.R. Ye, S. Geromanos, P. Tempst, T. Kirchhausen, J.E. Rothman, T. Söllner, "A rab protein is required for the assembly of SNARE complexes in the docking of transport vesicles," Cell, 78:937-48, 1994. (Cited in more than 60 publications through April 1996) Comments by Thomas Söllner, Memorial Sloan-Kettering Cancer Center, New York In 1993, James E. Rothman, the program chairman in cellular biochemistry and biophysics at Memorial Sloan-Kettering Cancer Center in New York, and his research team delineated the proteins involved in docking and fusion of synaptic vesicles with the presynaptic plasma membrane, a process essential for neurotransmission (T. Söllner et al., Cell, 75:409-18, 1993; Hot Papers, The Scientist, Sept. 4, 1995, page 15). Those findings led them to hypothesize that a specific pair of soluble NSF-attached protein (SNAP) receptors-so-called SNAREs-were responsible for targeting vesicles outside the neuronal synapse. PROTEIN POWER: Thomas Söllner helped to examine SNAREs. The researchers postulated that the unique pairing of a v-SNARE-the protein localized to transport vesicles-and its cognate t-SNARE protein-the one localized to target membranes-determines targeting specificity to vesicular transport. Assembled v/t-SNARE complexes bind additional proteins, the SNAPs, to the N-ethylmaleimide-sensitive fusion (NSF) protein. NSF hydrolyzes adenosine triphosphate (ATP) to disrupt this complex, initiating vesicle fusion by a still-unknown mechanism. Thomas Söllner, an assistant member in Rothman's laboratory and the senior author of this paper, says their research provides a critical piece of evidence for the SNARE hypothesis. He contends it proves that a specific v/t-SNARE complex forms in living cells, and reveals additional proteins involved in SNARE complex assembly. Using an NSF mutant in yeast, the researchers tested a prediction of the hypothesis-that SNARE complexes representing docked vesicles would accumulate when NSF was inactivated. "We used an antibody directed against a t-SNARE, which is localized to the Golgi apparatus, to precipitate any associated proteins," Sallner explains, "and, based upon our hypothesis, we expected to isolate a distinct v/t-SNARE complex originating from the endoplasmic reticulum [ER]." In other words, they expected to find ER-derived vesicles docked to the Golgi apparatus. "That's indeed what we found. The immunoprecipitate contained several proteins. Some of these proteins were already known; they had been identified as secretion-deficient yeast mutants, and had been localized to ER-Golgi transport vesicles. In addition, we found three new proteins whose function is still under investigation." Since then, Söllner states, Hugh Pelham's group at Cambridge University in the United Kingdom has isolated and characterized one of these new proteins, p14/Sft1p, by a different approach (D.K. Banfield et al., Nature, 375:806-9, 1995). Meanwhile, Söllner, Rothman, and their colleagues continue to investigate the other two SNARE proteins, p26 and p28. "This was the first time that such a v/t-SNARE complex had been purified outside of the neuronal synapse, illustrating the generality of the SNARE mechanism for vesicle docking and fusion," Sallner declares. "An additional discovery was that small GTP-binding proteins termed 'rab proteins' are necessary for the assembly of the SNARE complex, or for the activation of SNAREs," he adds. Söllner notes that vesicular transport is essential for a variety of processes. "For example, hormones and a variety of growth factors are released from cells by similar mechanisms, and it is known that alterations in such proteins can cause cancer."

Cell Biochemistry

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