Bovine heart mitochondnal succinate-ubiquinone reductase (SQR),which catalyses electron transfer from succinate to ubiquinone, is composed of a soluble succinate dehydrogenase (SDK) and a membrane anchoring protein fraction (QPs). QPs forms Q-binding site(s) and serves as a membrane anchor for SDK, converting it to a TTFA-sensitive SQR. This fraction contains three protein subunits named QPsl(lSkDa), QPs2 (13kDa), and QPs3 (llkDa). QPsl has been cloned and sequenced and its Q-binding domain located. The protein sequences of QPs2 and QPs3 are not yet available. Recently, pure QPs3 protein was isolated and a partial N-terminal amino acid sequence (42 residues) was determined. This enabled us to obtain the cDNA encoding QPs3 from a bovine heart cDNA library using PCR techniques. The QPs3 cDNA is 1150 bp with an open reading frame of 474 bp which encodes a 103amino acid mature QPs3 and a 55-amino acid presequence. The molecular weight of QPs3, calculated from the deduced amino acid sequence, is 10989 daltons. QPs3 is a very hydrophobic protein, with three transmembrane helices revealed by a hydropathy plot of the sequence. The deduced sequence was confirmed by 5 additional peptide sequences obtained from pure QPs3.