The nuclear‐encoded 18 kDa (IP) AQDQ subunit of bovine heart complex I is phosphorylated by the mitochondrial cAMP‐dependent protein kinase

Abstract

In bovine heart mitochondria a protein of M, 18 kDa, phosphorylated by mtPKA, is associated to the NADH-ubiquinone oxidoreductase in the inner membrane and is present in purified preparation of this complex. The 18 kDa phosphoprotein has now been isolated and sequenced. It is identified as the 18 kDa (IP) AQDQ subnnit of complex I, a protein of 133 amino acids with a phospborylation consensus site RVS at position 129-131.

Topology of the mitochondrial cAMP‐dependent protein kinase and its substrates

“…The phosphoprotein of 29 kDa, which is detected in intact mitochondria but not in the inner membrane fraction [1,2], is apparently a matrix protein. Work is in progress in our laboratory, along the lines reported in [6], to identify the mitochondrial proteins phosphorylated by mtPKA and clarify their eventual role.…”

“…The 42 kDa phosphoprotein was found to be loosely associated with NADH-ubiquinone oxidoreductase (complex I), ubiquinone-cytochrome c oxidoreductase (complex III), cytochrome c oxidase (complex IV) and FoFi ATP synthase (complex V) of the inner mitochondrial membrane, from which it was removed upon their purification [5]. The 18 kDa phosphoprotein copurified with complex I and resulted by sequence analysis to correspond to the nuclear-encoded 18 kDa (IP)AQDQ subunit of this complex [6]. Protein bands of 42, 18 and 6.5 kDa were, however, found to be phosphorylated also by the cAMP-dependent protein kinase of the cytosol (cPKA) [2].…”

Definition of the Nuclear Encoded Protein Composition of Bovine Heart Mitochondrial Complex I

“…For example, the 39-kDa subunit has a nucleotidebinding site (14); subunit SDAP is an acyl carrier protein (15,16); the 18-kDa subunit may be phosphorylated (17,18); and subunit B16.6 has been implicated in the interferon-␤-and retinoic acid-induced pathway of cell death (2). Thus, some subunits may have regulatory roles and be implicated in human diseases that arise from complex I dysfunction (1).…”

Effects of disrupting the 21 kDa subunit of complex I from Neurospora crassa

“…The first (and yet unique) mutation in nuclear-coded subunits of complex I involved in mitochondrial diseases was found in the AQDQ human homologue of the 21 kDa protein [10]. The patient presented with a multisystemic disorder with a fatal progressive phenotype, owing to a pathological duplication of five base pairs in the gene that altered the C-terminal region and abolished the putative phosphorylation site of the protein [10,11]. The fact that the patient was homozygous for the mutation and originated from two heterozygous parents [10] suggests that a ' loss-of-function ' phenotype is involved.…”

“…In fact, the first description of a human mutation affecting a nuclearcoded subunit of complex I has appeared recently [10]. The protein, called AQDQ, is phosphorylated by a mitochondrial cAMP-dependent kinase, although the role of this modification is unknown [11]. It belongs to the peripheral domain of complex I in Neurospora crassa [12] and is present in the iron-sulphur protein fragment of the bovine enzyme [13].…”