A partially assembled complex I in NAD4-deficient mitochondria of maize

The proton-translocating NADH:ubiquinone oxidoreductase (respiratory comple x I) consists of at least 32 subunits in higher plants, nine of which are m itochondrially encoded (NAD 1-7, NAD4L, NAD9). Complex I (CI) has been anal yzed from a mitochondrial mutant of maize, NCS2, that carries a deletion fo r the 3' end of the nad4 gene. Mitochondria from highly defective, near-hom oplasmic mutant plants have only trace amounts of the normal complex I. Ins tead, a reduced amount of a smaller complex, which also exhibits NADH dehyd rogenase activity, is detected on 'blue-native' polyacrylamide gels. Subuni ts of 76 kDa, 40 kDa and 55 kDa, as well as NAD7 and NAD9, have been identi fied in the subcomplex by their cross-reactivity with heterologous antisera . The corresponding subunits in Neurospora are localized in a 'peripheral a rm' of CI, which is known to assemble independently of a 'membrane arm'. Th e maize NCS2 CI subcomplex is loosely bound to the membrane and is missing several subunits that could be membrane components. Thus, the mutant CI sub complex may consist of a peripheral arm. A reduction in the steady-state le vels of NAD7 and NAD9 in NCS2 mitochondria occurs despite normal rates of b iosynthesis and there is a concomitant decrease of the nuclear encoded 76 k Da subunit. The reduction in CI-associated NADH dehydrogenase activity in t he nad4-deficient NCS2 mutant mitochondria is not associated with a compens atory increase in the activities or amounts of the putative 'exogenous' NAD (P)H dehydrogenases that are found in plant mitochondria.