Isolation of supernumerary yeast ATP synthase subunits e and i - Characterization of subunit i and disruption of its structural gene ATP18

Two subunits of the yeast ATP synthase have been isolated. Subunit e was fo und loosely associated to the complex. Triton X-100 at a 1% concentration r emoved this subunit from the ATP synthase, The N-terminal sequencing of sub unit i has been performed. The data are in agreement with the sequence of t he predicted product of a DNA fragment of Saccharomyces cerevisiae chromoso me XIII, The ATP18 gene encodes subunit i, which is 59 amino acids long and corresponds to a calculated mass of 6687 Da, Its pi is 9.73, It is an amph iphilic protein having a hydrophobic N-terminal part and a hydrophilic C-te rminal part. It is not apparently related to any subunit described in other ATP synthases, The null mutant showed low growth on nonfermentable medium. Mutant mitochondria display a low ADP/O ratio and a decrease with time in proton pumping after ATP addition. Subunit i is associated with the complex ; it is not a structural component of the enzyme but rather is involved in the oxidative phosphorylations. Similar amounts of ATP synthase were measur ed for wild-type and null mutant mitochondria. Because a fold less specific ATPase activity was measured for the null mutant than for the wild-type mi tochondria, we make the hypothesis that the observed decrease in the turnov er of the mutant enzyme could be linked to a proton translocation defect th rough F-0.