title: Membrane topology of the acyl-lipid desaturase from Bacillus subtilis.
abstract: The Bacillus subtilis acyl-lipid desaturase (Delta5-Des) is an iron-dependent integral membrane protein, able to selectively introduce double bonds into long chain fatty acids. Structural information on membrane-bound desaturases is still limited, and the present topological information is restricted to hydropathy plots or sequence comparison with the evolutionary related alkane hydroxylase. The topology of Delta5-Des was determined experimentally in Escherichia coli using a set of nine different fusions of N-terminal fragments of Delta5-Des with the reporter alkaline phosphatase (Delta5-Des-PhoA). The alkaline phosphatase activities of cells expressing the Delta5-Des-PhoA fusions, combined with site-directed mutagenesis of His residues identified in most desaturases, suggest that a tripartite motif of His essential for catalysis is located on the cytoplasmic phase of the membrane. These data, together with surface Lys biotinylation experiments, support a model for Delta5-Des as a polytopic membrane protein with six transmembrane- and one membrane-associated domain, which likely represents a substrate-binding motif. This study provides the first experimental evidence for the topology of a plasma membrane fatty acid desaturase. On the basis of our results and the presently available hydrophobicity profile of many acyl-lipid desaturases, we propose that these enzymes contain a new transmembrane domain that might play a critical role in the desaturation of fatty acids esterified in glycerolipids.
mesh_terms: D001345:Autoradiography; D001412:Bacillus subtilis / Q000201:enzymology*; D001483:Base Sequence; D015153:Blotting, Western; D017931:DNA Primers; D000090943:Delta-5 Fatty Acid Desaturase; D044943:Fatty Acid Desaturases / Q000737:chemistry* / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics
chemical_list: D017931:DNA Primers; D000090943:Delta-5 Fatty Acid Desaturase; D011994:Recombinant Proteins; D044943:Fatty Acid Desaturases