Export of the Cox2p C-terminal tail from the mitochondrial matrix:
folding inside and signaling from outside?
Scott A.
Saracco, Thomas D. Fox
Molecular Biology and Genetics, Cornell
University, 333 Biotech. Bldg., Ithaca, NY 14853, USA
Both the N-tail
and C-tail domains of Cox2p, one of the mitochondrially synthesized core
subunits of cytochrome c oxidase, are translocated through the
mitochondrial inner membrane to the intermembrane space (IMS). Following
export of the N-tail, a leader peptide is removed in the IMS (the
trans side). In a selection for mutants defective in C-tail
export, we obtained multiple alleles of the nuclear genes COX18,
PNT1, and MSS2 which we found to encode interacting proteins
located on the inner membrane. These proteins are required for C-tail
export but not for N-tail export, demonstrating that the two Cox2p
domains are translocated by genetically distinct mechanisms. We also
obtained cis-acting mutations blocking C-tail export that affect
pre-Cox2p itself. Two of these, Y130N and F188T, change residues in the
Cox2p C-tail that point towards each other in the hydrophobic core of
this domain. Pseudorevertants of F188T all had missense mutations that
would tend to fill this hydrophobic core. These results demonstrate that
the highly acidic character of the C-tail is not sufficient to signal
its export, and suggest that at least partial folding of the C-tail
domain in the matrix may be required prior to export. The third
cox2 mutation was N15I, which destroys the leader peptide
cleavage site. Thus it appears that proteolytic cleavage on the
trans side of the inner membrane is a prerequisite for subsequent
export of the folded C-tail from the matrix.
Return to YGM 2002 Home at SGD