Syntaxin family SNARE proteins are integral membrane proteins that belong to Q-SNAREs i. e. they contain a glutamine at the central layer of the SNARE motif bundle. In addition to the SNARE motif, syntaxins have an N-terminal domain that is composed of three short helixes and a C-terminal transmembrane domain that is followed by a very short hydrophilic tail. S. cerevisiae expresses two highly homologous syntaxins Sso1p and Sso2p that both mediate membrane fusion during exocytosis at the plasma membrane. The Sso1p the N-terminal domain has been shown to interact with the SNARE motif and regulate the rate of SNARE complex assembly. Together, Sso1p and Sso2p perform an essential function in vegetatively growing haploid and diploid cells where they interact with plasma membrane SNARE proteins Sec9p, Snc1p and Snc2p. However, in meiotic diploid cells there is a specific Dasatinib abmole bioscience requirement for Sso1p for de novo formation of the prospore membrane during meiosis. The functional difference for Sso1p and Sso2p in meiotic cells is not explained by transcriptional regulation, or differences in expression levels. Both proteins are expressed at similar level in meiotic cells, localize to the prospore membrane, and swapping of promoters between SSO1 and SSO2 does not render Sso2p functional in prospore membrane formation. The two N-terminal a-helices Ha and Hb of Sso1p are important for its function during meiosis. In addition to the specific requirement of Sso1p, in sporulating cells the Q-SNARE Sec9p is replaced by a homologous protein Spo20. Recent results indicate that phosphatidic acid and PIP2 are important for membrane fusion during prospore membrane formation. However, the signals that regulate the activity of Sso1p and the initiation of meiotic SNARE complex formation are unknown. Post-translational modifications are central modifiers of protein activity. Mass spectrometry studies have revealed in vivo phosphorylation sites in the amino terminal part of Sso1p and Sso2p. In this study we set out to establish the contribution of these phosphoamino acids on the functional regulation of Sso1 and Sso2 proteins. In addition, we tested, whether, in analogy to meiosis and sporulation, also pseudohyphal and invasive growth, two nutritionally regulated cell differentiation processes display differential requirements for Sso1p and Sso2p. In addition, serine 79 was previously reported as an in vivo phosphorylation site in Sso1p. Subsequent analysis showed that S79 phosphorylation reduced participation of Sso1p in haploid cell SNARE complexes. These amino acids represent potential regulatory means to modulate Sso protein in vivo function and differentiate between these proteins during sporulation. The structure of a cytosolic fragment of Sso1p has been determined. This structure is missing the very amino-terminus that contains several phosphorylation sites in Sso1p and the homologous Sso2p. The amino-terminal peptides of several syntaxins do not refract well in crystals. This suggests that even when present in the analyzed protein the peptide is unstructured in monomeric syntaxins.