SULT4A1 is an enigma within the cytosolic sulfotransferase (SULT) gene family. Unlike other SULTs, SULT4A1 is selectively expressed in neurons in vertebrate brain and is highly conserved from mammals to fish suggesting an important conserved function. The human SULT4A1 gene (chr. 22q13.3), has been identified as a susceptibility gene for schizophrenia and is deleted in select cases of Phelan-McDermid (PMS) syndrome, an autism spectrum disorder. No drug/xenobiotic sulfating activity with SULT4A1 has been reported. All SULT4A1 isoforms lack 15 aa in loop3 that forms the top of the substrate binding pocket in other SULT isoforms. The missing 15aa help create a putative interaction site for protein substrates. Mouse SULT4A1-knockout (KO) models generated in our laboratory revealed SULT4A1's role in several vital neuronal functions. Homozygous SULT4A1-KO mice lack detectable SULT4A1 in brain and develop severe movement tremor and gait problems with absence seizures starting post-natal day 7-8. Pups show decreased weight gain, increasing immobility with tremor and spasticity, and usually die within 4-8 weeks. Heterozygous littermates have decreased SULT4A1 CNS expression and appear normal. This is the first mammalian model demonstrating severe neurological problems associated with loss of SULT4A1. SULT4A1 protein is expressed in CNS neurons with a unique subcellular distribution in mitochondrial, microsomal and cytosolic fractions but not nuclei. Our data supports the hypothesis that SULT4A1 is acting as a chaperone/regulatory protein in neurons via SULT4A1-protein interactions with a potential unique protein sulfation activity. SULT4A1 expression increases mitochondrial respiration and protects mouse neurons from H2O2 toxicity. In neurons, SULT4A1 interacts with proteins involved in mitochondria respiration, and other cellular pathways. The neuronal and stress protective functions of SULT4A1, and its unique subcellular localization provide the backdrop to investigate the biochemical, structural and functional properties of SULT4A1 in the genetically tractable yeast model, S. cerevisiae. SULT4A1 expressed in yeast displays a similar subcellular protein distribution as observed in neurons. Also, SULT4A1 expression stimulates yeast colony formation and displays the protective function to oxidative stress induced by H2O2. Therefore, two specific aims are proposed: Aim 1. To investigate SULT4A1 cellular function and determine SULT4A1 activity as a protein SULT in yeast. Preliminary data in yeast shows that the protective function and distribution of SULT4A1 is conserved. We propose that the highly conserved residues around the open catalytic pocket are responsible for protein interactions and potentially protein sulfation activity. Aim 2. To investigate the role of the SULT4A1 phosphorylation in protein-protein interactions. Co-immunoprecipitation studies will identify SULT4A1 protein interactors in yeast. Select SULT4A1 mutants of the conserved Thr phosph...