Membrane Topology of the Acyl-Lipid Desaturase from Bacillus subtilis

Abstract

The Bacillus subtilis acyl-lipid desaturase (⌬5-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 ⌬5-Des was determined experimentally in Escherichia coli using a set of nine different fusions of N-terminal fragments of ⌬5-Des with the reporter alkaline phosphatase (⌬5-Des-PhoA). The alkaline phosphatase activities of cells expressing the ⌬5-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 ⌬5-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.The fatty acyl desaturases encompass a family of enzymes, representatives of which are found in all lower eukaryotes, plants and animals (for a review see Ref. 1), and some prokaryotes such as cyanobacteria (2) and bacilli (3), which have the function of introducing double bonds into fatty acyl chains.Although they all utilize molecular oxygen and reducing equivalents obtained from an electron transport chain, and the basic mechanism of the desaturation reaction may be very similar in all cases (4), fatty acid desaturases can be classified into three main subfamilies (1): (i) the soluble acyl carrier protein desaturases that introduce double bonds into fatty acids esterified to acyl carrier protein and are found in the stroma of plant plastids (4); (ii) the acyl-lipid desaturases, which are membranebound enzymes associated with the endoplasmic reticulum (1), the plant chloroplast membrane (5), the cytoplasmic membrane of some bacilli (6), and the plasmatic and thylacoid membranes of cyanobacteria (7) that desaturate fatty acids esterified in glycerolipids; (iii) the acyl-CoA desaturases, which introduce double bonds into fatty acids esterified to CoA (8) and are associated to the endoplasmic reticulum membrane of animals and fungi (1).The soluble and the membrane-bound desaturases show different consensus motifs (for a review, see 4). Database searching for these motifs reveals that they belong to two distinct multifunctional classes of iron-dependent enzymes, each of which includes desaturases, hydroxylases, and epoxidases that act on fatty acids ...

Domain Swapping Localizes the Structural Determinants of Regioselectivity in Membrane-bound Fatty Acid Desaturases of Caenorhabditis elegans

“…Therefore, these two domains combined appear to be responsible in large part to the determination of regioselectivity in the -3 and "⌬12" desaturases of C. elegans. The importance of domain G in C. elegans desaturase activity is consistent with previous studies indicating that removal or replacement of sequence at the carboxyl terminus of desaturases can abolish activity (13). The results from our work on C. elegans desaturases extend this observation to show that the carboxyl terminus is actually an important determinant of regioselectivity.…”

“…This work hinted at the importance of the carboxyl terminus of desaturases in determining specific regiochemical and/or substrate characteristics of these enzymes. Additionally the inactivity of a carboxyl-terminal deletion mutant of the Bacillus subtilis ⌬5 desaturase supports the importance of domain G in desaturase function (13).…”

“…On this basis, a model for the membrane topology of this superfamily has been proposed as depicted in Fig. 1 (9,11,12), although an alternative topology has been suggested (13).…”

A Small Membrane-peripheral Region Close to the Active Center Determines Regioselectivity of Membrane-bound Fatty Acid Desaturases from Aspergillus nidulans

“…However, the majority of desaturase enzymes reside within membranes, and up to now the principles of membrane-bound desaturase specificity are not understood. Moreover, structural information on membrane-bound desaturases is limited, so only hydropathy and topology analyses are available, predicting the formation of four transmembrane domains and either one or two membrane-peripheral protein domains (7)(8)(9). Three highly conserved histidine-rich motifs are essential for catalysis (1,10), and it was proposed that these histidine clusters coordinate two iron atoms in the active site (11).…”

Characterization of membrane-bound fatty acid desaturases

“…From topological point of view, each desaturase crosses the membrane four times in two hydrophobic domains and all three His boxes are on membrane of endoplasmatic reticulum towards cytosol (Stuckey et al 1990;Shanklin and Cahoon 1998). This corresponds to suppose that His (Diaz et al 2002).…”