Three subunit proteins of membrane enzymes in mitochondria of Neurospora crassa contain a pantothenate derivative

NADH dehydrogenase in Neurospora crassa contains myristic acid covalently linked to the ND5 subunit peptide

The mitochondrial, proton-pumping NADH:ubiquinone oxidoreductase consists of at least 35 subunits whose synthesis is divided between the cytosol and mitochondria; this complex I catalyzes the first steps of mitochondrial electron transfer and proton translocation. Radiolabel from [(3)H]myristic acid was incorporated by Neurospora crassa into the mitochondrial-encoded, approximately 70 kDa ND5 subunit of NADH dehydrogenase, as shown by immunoprecipitation. This myristate apparently was linked to the peptide through an amide linkage at an invariant lysine residue (Lys546), based upon analyses of proteolysis products. The myristoylated lysine residue occurs in the predicted transmembrane helix 17 (residues 539-563) of ND5. A consensus amino acid sequence around this conserved residue exists in homologous subunits of NADH dehydrogenase. Cytochrome c oxidase subunit 1, in all prokaryotes and eukaryotes, contains this same consensus sequence surrounding the lysine which is myristoylated in N. crassa.

Presence of an acyl carrier protein in NADH:ubiquinone oxidoreductase from bovine heart mitochondria

The amino-acid sequence of a subunit of NADH:ubiquinone oxidoreductase from bovine heart mitochondria has been determined and is closely related to those of acyl carrier proteins that are involved in fatty acid biosynthesis in Escherichia coli and plants. Evidence for the presence of covalently attached pantetheine-4'-phosphate in the bovine protein has been obtained by determination of the molecular mass of the isolated subunit by electrospray mass spectrometry, before and after incubation of the protein at alkaline pH under reducing conditions. This decreased the molecular mass from 10,751.6 to 10,449.4, a difference of 302.2 mass units; the value calculated from the protein sequence with one covalently attached pantetheine-4'-phosphate is 10,449.8. The acyl group which is removed by alkaline reduction, appears to be attached via a thioester linkage. By analogy with the bacterial protein it is likely that the attachment site of the pantetheine-4-phosphate is serine-44, which is found in a highly conserved region of the sequence. At present the function of the acyl carrier protein in mitochondrial complex I is not understood.

Pantothenic acid in health and disease

In summary, the vitamin pantothenic acid is an integral part of the acylation carriers, CoA and acyl carrier protein (ACP). The vitamin is readily available from diverse dietary sources, a fact which is underscored by the difficulty encountered in attempting to induce pantothenate deficiency. Although pantothenic acid deficiency has not been linked with any particular disease, deficiency of the vitamin results in generalized malaise clinically. In view of the fact that pantothenate is required for the synthesis of CoA, it is surprising that tissue CoA levels are not altered in pantothenate deficiency. This suggests that the cell is equipped to conserve its pantothenate content, possibly by a recycling mechanism for utilizing pantothenate obtained from degradation of pantothenate-containing molecules. Although the steps involved in the conversion of pantothenate to CoA have been characterized, much remains to be done to understand the regulation of CoA synthesis. In particular, in view of what is known about the in vitro regulation of pantothenate kinase, it is surprising that the enzyme is active in vivo, since factors that are known to inhibit the enzyme are present in excess of the concentrations known to inhibit the enzyme. Thus, other physiological regulatory factors (which are largely unknown) must counteract the effects of these inhibitors, since the pantothenate-to-CoA conversion is operative in vivo. Another step in the biosynthetic pathway that may be rate limiting is the conversion of 4'-phosphopantetheine (4'-PP) to dephospho-CoA, a step catalyzed by 4'-phosphopantetheine adenylyl-transferase. In mammalian systems, this step may occur in the mitochondria or in the cytosol. The teleological significance of these two pathways remains to be established, particularly since mitochondria are capable of transporting CoA from the cytosol. Altered homeostasis of CoA has been observed in diverse disease states including starvation, diabetes, alcoholism, Reye syndrome (RS), medium-chain acyl CoA dehydrogenase deficiency, vitamin B12 deficiency, and certain tumors. Hormones, such as glucocorticoids, insulin, and glucagon, as well as drugs, such as clofibrate, also affect tissue CoA levels. It is not known whether the abnormal metabolism observed in these conditions is the result of altered CoA metabolism or whether CoA levels change in response to hormonal or nonhormonal perturbations brought about in these conditions. In other words, a cause-effect relation remains to be elucidated. It is also not known whether the altered CoA metabolism (be it cause or result of abnormal metabolism) can be implicated in the manifestations of a disease. Besides CoA, pantothenic acid is also an integral part of the ACP molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurospora mitochondria contain an acyl-carrier protein

Mitochondria of Neurospora crassa were found to contain a protein which was labelled with [14C]pantothenic acid and which carried an acyl group. This protein, when purified 6000-fold, closely resembled the bacterial and chloroplast acyl-carrier protein(s) [ACP(s)] in its physical and chemical properties. The predominant acyl group esterified to the purified protein was 3-hydroxytetradecanoate, as determined by gas chromatographic mass spectrometry. The amino acid sequence of the tryptic peptide carrying the 4'-phosphophantetheine moiety showed a high degree of sequence similarity to the analogous bacterial and chloroplast ACP peptide sequences. The possible functions of this ACP in lipid metabolism are discussed in view of the fact that Neurospora has a separate cytoplasmic enzyme complex which carries out the de novo biosynthesis of fatty acids.

Preparation of high-specific-activity D-[3-3H]pantothenic acid

High-specific-activity D-[3-3H]pantothenic acid (5 Ci/mmol) was prepared from commercially available beta-[3-3H]alanine employing Escherichia coli strain DV1 (panD2 pan F1). This strain is defective in beta-alanine synthesis and pantothenate uptake, and under appropriate growth conditions converted 85 to 90% of the input beta-[3-3H]alanine to extracellular D-[3-3H]pantothenate. The radiolabeled vitamin was purified from the medium by thin-layer chromatography followed by reverse-phase high-performance liquid chromatography. The overall yield of D-[3-3H]pantothenic acid was 30% and radiochemical purity was greater than 99%.

A pantothenate derivative is covalently bound to mitochondrial proteins in Neurospora crassa

The presence of protein-bound pantothenate in Neurospora crassa was investigated by labelling a pantothenate auxotroph (pan-2) with [14C]pantothenate and examining mycelial homogenates on dodecyl sulfate/polyacrylamide gels. Five peaks of radioactivity were found, with apparent molecular masses of 200, 140, 22, 19, and 9 kDa. The 200-kDa peak was identified as fatty acid synthetase, based on its absence in a fatty acid synthetase mutant. The 22-kDa and 19-kDa peaks co-purified with mitochondrial markers on sucrose gradients. When purified mitochondria were fractionated, the 19-kDa protein was associated with the inner membrane and the 22-kDa protein was enriched in the soluble mitochondrial fraction. The label was quantitatively recovered from the mitochondrial proteins as 4'-phosphopantetheine after mild alkaline hydrolysis. Although the function of this post-translational modification of mitochondrial proteins is not known, several possibilities are discussed: the 4'-phosphopantetheine may act as a carrier group in an enzymatic reaction, or it may perform a regulatory function as part of an enzyme complex.