Passage
Presynaptic nerve terminals release neurotransmitters via synaptic vesicle exocytosis. This action is mediated by the membrane-bound target-SNARE (t-SNARE) proteins syntaxin and SNAP-25, and the vesicle-associated SNARE (v-SNARE) protein synaptobrevin. Membrane fusion is initiated when the t-SNAREs and v-SNAREs form an α-helix bundle known as the core-trans-complex, achieved by zipping SNARE proteins together to form a more stable cis-complex (Figure 1) .
Figure 1 Formation of the core-trans-complex and subsequent membrane fusionThe cis-complex contains the zero ionic layer, the main site of interaction between complexed proteins. Within the zero ionic layer, arginine on synaptobrevin coordinates with carbonyl groups present on the other residues shown in Table 1. The zero ionic layer is buried within leucine zipper domains, which act as a shield to solvent molecules.Table 1 Composition of cis-Complex Zero Ionic Layer
An early step in cis-complex disassembly is destabilization by the ATPase N-ethylmaleimide-sensitive factor (NSF) , which initiates unzipping by breaking flanking leucine zipper regions, leading to vesicle reuptake. Movement of NSF to the cell membrane is mediated by the cytoplasmic protein α-SNAP, which must first bind the N-terminal domain of syntaxin.To determine the effect of vesicle-bound synaptobrevin on the binding of α-SNAP to syntaxin, increasing amounts of recombinant His-tagged α-SNAP were added to a constant amount of syntaxin affixed to glutathione-agarose beads with or without synaptobrevin. After incubation, bound proteins were recovered and quantitatively analyzed by immunoblotting. Results are shown in Figure 2.
Figure 2 α-SNAP binding to syntaxin in the absence (square) or presence (circle) of synaptobrevin
Adapted from Mcmahon HT, Südhof TC. Synaptic core complex of synaptobrevin, syntaxin, and SNAP25 forms high affinity alpha-SNAP binding site. J Biol Chem. 1995;270(5) :2213-7.
-Which amino acid is LEAST likely to be found in normally functioning binding domains of t-SNARE and v-SNARE proteins?

Question

Passage
Presynaptic nerve terminals release neurotransmitters via synaptic vesicle exocytosis. This action is mediated by the membrane-bound target-SNARE (t-SNARE) proteins syntaxin and SNAP-25, and the vesicle-associated SNARE (v-SNARE) protein synaptobrevin. Membrane fusion is initiated when the t-SNAREs and v-SNAREs form an α-helix bundle known as the core-trans-complex, achieved by zipping SNARE proteins together to form a more stable cis-complex (Figure 1) .

Figure 1 Formation of the core-trans-complex and subsequent membrane fusionThe cis-complex contains the zero ionic layer, the main site of interaction between complexed proteins. Within the zero ionic layer, arginine on synaptobrevin coordinates with carbonyl groups present on the other residues shown in Table 1. The zero ionic layer is buried within leucine zipper domains, which act as a shield to solvent molecules.Table 1 Composition of cis-Complex Zero Ionic Layer

An early step in cis-complex disassembly is destabilization by the ATPase N-ethylmaleimide-sensitive factor (NSF) , which initiates unzipping by breaking flanking leucine zipper regions, leading to vesicle reuptake. Movement of NSF to the cell membrane is mediated by the cytoplasmic protein α-SNAP, which must first bind the N-terminal domain of syntaxin.To determine the effect of vesicle-bound synaptobrevin on the binding of α-SNAP to syntaxin, increasing amounts of recombinant His-tagged α-SNAP were added to a constant amount of syntaxin affixed to glutathione-agarose beads with or without synaptobrevin. After incubation, bound proteins were recovered and quantitatively analyzed by immunoblotting. Results are shown in Figure 2.

Figure 2 α-SNAP binding to syntaxin in the absence (square) or presence (circle) of synaptobrevin
Adapted from Mcmahon HT, Südhof TC. Synaptic core complex of synaptobrevin, syntaxin, and SNAP25 forms high affinity alpha-SNAP binding site. J Biol Chem. 1995;270(5) :2213-7.
-Which amino acid is LEAST likely to be found in normally functioning binding domains of t-SNARE and v-SNARE proteins?

Quizplus · Accepted Answer

11ecba2d_0de4_dda3_a3bf_1dd81c2b36b3_MD0008_00 As mentioned in the passage, the interacting protein domains are α-helical. Along with β-sheets, α-helices are common secondary structures found in proteins. They consist of single strands of protein arranged in spirals, with side groups extending outward. These helices include 3.6 amino acids per turn and are stabilized by hydrogen bonds between carbonyl (C=O) and amide (N-H) groups at the n and (n + 4) positions, respectively. Proline and glycine are often found in loops and linker regions between α-helices and β-sheets, particularly at sharp turns. Proline is unlikely to be found within an α-helix because it is structurally rigid and introduces a kink in the chain that is useful for sharp turns but disrupts α-helices. In addition, proline's amide group is contained within its ring structure and is unable to participate in hydrogen bonds. Glycine has the smallest side group of all the amino acids (a hydrogen atom) and is quite flexible. Although ideal for sharp turns, this flexibility causes α-helices to be too "floppy" and disrupts the structure. (Choices A, B, and C) Both polar and nonpolar amino acids are found in α-helices. Polar amino acids such as asparagine and threonine are more likely to be found in regions exposed to water. Nonpolar amino acids such as tryptophan are more likely to be found within transmembrane regions, buried within a folded protein, or at sites of protein-protein interaction. Proline and glycine are the only amino acids that are not commonly found in α-helices. Educational objective: α-helices have 3.6 amino acids per turn and are stabilized by hydrogen bonds between carbonyl and amide groups. Of the amino acids, proline is least likely to be found within α-helices and is more often found in linker regions or at sharp turns due to its structural rigidity. Glycine also tends to disrupt helices due to its excessive flexibility.

Passage Presynaptic Nerve Terminals Release Neurotransmitters Via Synaptic Vesicle Exocytosis. This