We determined the primary structure of a 9.6-kDa subunit of the respiratory chain NADH : ubiquinone reductase (complex 1) from Neurospora crassa mitochondria and found a close relationship between this subunit and the bacterial or chloroplast acyl-carrier protein. The degree of sequence identity amounts to 80% in a region of 19 residues around the serine to which the phosphopantetheine is bound. The N-terminal presequence of the subunit has the characteristic features of a mitochondrial import sequence. We cultivated the auxotroph pan-2 mutant of N . crassa in the presence of [14C]pantothenate and recovered all radioactivity incorporated into mitochondrial protein in the 9.6-kDa subunit of complex I. We cultivated N . crassa in the presence of chloramphenicol to accumulate the nuclear-encoded peripheral arm of complex I. This pre-assembled arm also contains the 9.6-kDa subunit. These results demonstrate that an acyl-carrier protein with pantothenate as prosthetic group is a constituent part of complex I in N . crassa.In a series of reports, Brody and coworkers [I-41 have demonstrated that mitochondria of Neurospora crassa and other microorganisms contain an acyl-carrier protein (ACP) with phosphopantetheine as prosthetic group. This mitochondrial ACP can mediate de novo fatty acid synthesis, independent of the fatty acid synthetase complex present in the cytoplasm [4]. The role of the mitochondrial synthetic pathway, however, remained unclear, and the authors considered the possibility that the pathway exists to satisfy special needs of lipids of the mitochondrion itself [4]. Antigenically similar proteins of similar apparent molecular masses were also found in the mitochondria of yeast, pea leaves and potato tubers, but not in animal mitochondria [3]. In order to isolate the ACP, the authors had to dissolve the N. crassa mitochondria in 6 M guanidine hydrochloride [2].The respiratory chain complex of NADH : ubiquinone oxidoreductase (complex I) in N . crassa mitochondria is an assembly of some 30 different subunits. Most of these are nuclear encoded and imported from the cytoplasm. In N . crassu, at least six (seven in mammals) are encoded and translated in the mitochondrion. Only a few subunits are directly involved in the binding of the substrates NADH and ubiquinone, the internal redox groups FMN and four (EPRdetectable) iron-sulfur clusters. The functions of the many additional subunits which do not appear to take part directly in electron transport remain largely unknown (for a review,

No abstract

The primary structure of the 49 K subunit of the respiratory chain NADH:ubiquinone reductase (complex I) from Neurospora crassa was determined by sequencing cDNA, genomic DNA and the N-terminus of the mature protein. The sequence lengths correlate to a molecular mass of 54,002 daltons for the preprotein and 49,239 daltons for the mature protein. The presequence consists of 42 amino acids of typical composition for sequences which target nuclear-encoded proteins into mitochondria. The mature protein consists of 436 amino acids and shows 64% similarity to a 49 K subunit of bovine heart NADH:ubiquinone reductase and 33% to a predicted translation product of an open reading frame in the chloroplast DNAs of Marchantia polymorpha and Nicotiana tabacum. Evidence for an iron-sulfur cluster in the subunit is discussed.

No abstract