Serine palmitoyltransferase (SPT) catalyzes the first committed step in sphingolipid biosynthesis. In yeast, SPT is composed of a heterodimer of 2 highly-related subunits, Lcb1p and Lcb2p, and a third subunit, Tsc3p, which increases enzyme activity markedly and is required for growth at elevated temperatures. Higher eukaryotic orthologs of Lcb1p and Lcb2p have been identified, but SPT activity is not highly correlated with coexpression of these subunits and no ortholog of Tsc3p has been identified. Here, we report the discovery of 2 proteins, ssSPTa and ssSPTb, which despite sharing no homology with Tsc3p, each substantially enhance the activity of mammalian SPT expressed in either yeast or mammalian cells and therefore define an evolutionarily conserved family of low molecular weight proteins that confer full enzyme activity. The 2 ssSPT isoforms share a conserved hydrophobic central domain predicted to reside in the membrane, and each interacts with both hLCB1 and hLCB2 as assessed by positive split ubiquitin 2-hybrid analysis. The presence of these small subunits, along with 2 hLCB2 isofoms, suggests that there are 4 distinct human SPT isozymes. When each SPT isozyme was expressed in either yeast or CHO LyB cells lacking endogenous SPT activity, characterization of their in vitro enzymatic activities, and long-chain base (LCB) profiling revealed differences in acyl-CoA preference that offer a potential explanation for the observed diversity of LCB seen in mammalian cells.

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The autosomal dominant peripheral sensory neuropathy HSAN1 results from mutations in the LCB1 subunit of serine palmitoyltransferase (SPT). Serum from patients and transgenic mice expressing a disease-causing mutation (C133W) contain elevated levels of 1-deoxysphinganine (1-deoxySa), which presumably arise from inappropriate condensation of alanine with palmitoyl-CoA. Mutant heterodimeric SPT is catalytically inactive. However, mutant heterotrimeric SPT has ϳ10 -20% of wild-type activity and supports growth of yeast cells lacking endogenous SPT. In addition, long chain base profiling revealed the synthesis of significantly more 1-deoxySa in yeast and mammalian cells expressing the heterotrimeric mutant enzyme than in cells expressing wild-type enzyme. Wild-type and mutant enzymes had similar affinities for serine. Surprisingly, the enzymes also had similar affinities for alanine, indicating that the major affect of the C133W mutation is to enhance activation of alanine for condensation with the acyl-CoA substrate. In vivo synthesis of 1-deoxySa by the mutant enzyme was proportional to the ratio of alanine to serine in the growth media, suggesting that this ratio can be used to modulate the relative synthesis of sphinganine and 1-deoxySa. By expressing SPT as a single-chain fusion protein to ensure stoichiometric expression of all three subunits, we showed that GADD153, a marker for endoplasmic reticulum stress, was significantly elevated in cells expressing mutant heterotrimers. GADD153 was also elevated in cells treated with 1-deoxySa. Taken together, these data indicate that the HSAN1 mutations perturb the active site of SPT resulting in a gain of function that is responsible for the HSAN1 phenotype. Serine palmitoyltransferase (SPT)3 catalyzes the committed and rate-limiting step in sphingolipid biosynthesis. Until recently, it was believed that there was a single isozyme of this enzyme containing two subunits, LCB1 and LCB2, with the catalytic domain located at the interface and composed of residues from each subunit. However, Hornemann et al. showed that there was a second isoform of LCB2 (1). More recently, we identified two highly related isoforms of a third subunit, ssSPTa and ssSPTb, which are essential for maximal enzymatic activity (2). Thus, there are at least four potential SPT isozymes, each containing a common hLCB1 subunit and one of each of the two hLCB2 and ssSPT subunits. Examination of the acyl-CoA preferences of the hLCB1/LCB2a and hLCB1/LCB2b heterodimers and the hLCB1/LCB2/ssSPT heterotrimers showed that both the LCB2 isoform and the ssSPT isoform conferred different acyl-CoA preferences (2, 3), revealing a previously unappreciated complexity in SPT substrate selectivity, and the corresponding long chain base (LCB) products.Mutations in the hLCB1 subunit of SPT have been demonstrated to cause the autosomal dominant late-onset hereditary sensory neuropathy, type I (HSAN1) (4 -6). In addition, an HSAN1-like phenotype is seen in transgenic mice overexpressing the LCB1 C133W mutation (7). Beca...

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It has been suggested previously that copper transport in Escherichia coli is mediated by the products of at least six genes, cutA, cutB, cutC, cutD, cutE, and cutF. A mutation in one or more of these genes results in an increased copper sensitivity ( Copper is an essential heavy metal trace element, which plays a vital role in the growth and physiology of aerobic organisms; however, excess of this metal results in cell death.