T. Papakonstantinou et al., EACH OF 3 POSITIVELY-CHARGED AMINO-ACIDS IN THE C-TERMINAL REGION OF YEAST MITOCHONDRIAL ATP SYNTHASE SUBUNIT-8 IS REQUIRED FOR ASSEMBLY, Biochimica et biophysica acta, 1144(1), 1993, pp. 22-32
Each of three conserved positively-charged residues in the C-terminal
region of subunit 8 of yeast (Saccharomyces cerevisiae) mitochondrial
ATP synthase was replaced with isoleucine. The assembly and functional
properties of the resulting variants (substituted at Arg-37, Arg-42 a
nd Lys-47) were examined using in-vitro systems to assay import into i
solated mitochondria and to monitor assembly into ATP synthase, as wel
l as an in-vivo rescue system using host yeast cells lacking endogenou
s subunit 8. Each such variant was found to be impaired in assembly in
vitro, after import in the form of a chimaeric protein bearing a lead
er sequence with mitochondrial targeting function. Import precursors b
earing a duplicated-leader sequence, engendering enhanced delivery to
mitochondria of the passenger variant subunit-8 proteins, enabled asse
mbly of the (Lys-47 --> Ile) variant to be detected in vitro but not t
hat of (Arg-37 --> Ile) or (Arg-42 --> Ile) variants. The respiratory
growth of subunit 8-deficient host cells could be rescued with the (Ly
s-47 --> Ile) variant expressed allotopically in the nucleus. Such res
cued cells were found to have an enhanced growth rate (comparable to t
hat produced by non-mutagenized parental subunit 8) when delivered to
mitochondria with the duplicated-leader sequence, as compared to the s
ingle-leader sequence. This confirms that the impediment in the (Lys-4
7 --> Ile) variant lies in the efficiency of its assembly, rather than
a functional defect, as such, arising from the loss of that positive
charge. In contrast, host cells were unable to be rescued by the (Arg-
37 --> Ile) and (Arg-42 --> Ile) variants, even when they were endowed
with the duplicated leader sequence. It is concluded that the positiv
ely-charged C-terminal domain of subunit 8, common to fungal and mamma
lian homologues of this protein, plays a key role in its assembly into
mitochondrial ATP synthase.