Conservation and Divergence of Threonine 254 among Different Syntaxin Orthologs

<div><p>(A) Proposed model of SNARE complex assembly and disassembly in a synaptic vesicle cycle (adapted from [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050072#pbio-0050072-b005" target="_blank">5</a>]). (1) Synaptobrevin forms a partial <i>trans</i> SNARE complex with syntaxin 1A and SNAP-25. (2) By zippering in an N- to C-termini direction, the SNARE proteins form a <i>trans</i> complex and bring the synaptic vesicle close to the plasma membrane. SNARE-mediated synaptic vesicle exocytosis occurs either spontaneously (3) or evoked by Ca²⁺ (4). (5) <i>cis</i> SNARE complexes are thought to be disassembled by NSF ATPase prior to vesicle recycling. ER, endoplasmic reticulum; PM, plasma membrane; SV, synaptic vesicle.</p> <p>(B) Alignment of amino acids (aa) around position T254 in the <i>Drosophila</i> syntaxin 1A or equivalent residues in syntaxin orthologs from a variety of animals, yeast, and the plant <i>Arabidopsis</i>. The top panel shows a cartoon of syntaxin 1A and the region of the alignment. Syntaxins are organized as “plasma membrane” or “intracellular compartments” according to their cellular distributions. With the exception of syntaxin 4, most plasma membrane syntaxins are known to function in presynaptic terminals or neurosecretory cells for Ca²⁺-regulated exocytosis. Note that T254 is highly conserved among “presynaptic” syntaxin 1A, 2, and 3A molecules. We call all other syntaxin orthologs shown here “constitutive” syntaxins because they are used for constitutive secretion on the plasma membrane (PM) and intracellular compartments, such as the endosome and the lysosome, the <i>cis</i> and <i>trans</i> Golgi network (Golgi network), and endoplasmic reticulum (ER) [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050072#pbio-0050072-b001" target="_blank">1</a>]. The yeast plasma membrane syntaxin orthologs SSO1 and SSO2, and syntaxins 4 and 131 from <i>Arabidopsis</i> are also shown here. (A more complete alignment can be see in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050072#pbio-0050072-sg001" target="_blank">Figure S1</a>.) Unlike the synaptic syntaxins, syntaxin 4 and most syntaxin 11s have a valine (V) at the 254 equivalent position, syntaxins 6, 7, 12, 16, and 17 a leucine (L), and syntaxin 5 an isoleucine (I). The isoleucine found in the <i>syx^3–69</i> mutant resembles some of the wild-type syntaxin orthologs used for constitutive secretion. The core complex layers from 0 to +8 are identified at the bottom. The aa sequence was obtained from the NIH's National Center for Biotechnology Information (NCBI; <a href="http://www.ncbi.nlm.nih.gov" target="_blank">http://www.ncbi.nlm.nih.gov</a>) and aligned using the software DNAStar.</p>