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2016 ; 473
(18
): 2813-29
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Import of a major mitochondrial enzyme depends on synergy between two distinct
helices of its presequence
#MMPMID27422783
Kalef-Ezra E
; Kotzamani D
; Zaganas I
; Katrakili N
; Plaitakis A
; Tokatlidis K
Biochem J
2016[Sep]; 473
(18
): 2813-29
PMID27422783
show ga
Mammalian glutamate dehydrogenase (GDH), a nuclear-encoded enzyme central to
cellular metabolism, is among the most abundant mitochondrial proteins
(constituting up to 10% of matrix proteins). To attain such high levels, GDH
depends on very efficient mitochondrial targeting that, for human isoenzymes
hGDH1 and hGDH2, is mediated by an unusually long cleavable presequence (N53).
Here, we studied the mitochondrial transport of these proteins using isolated
yeast mitochondria and human cell lines. We found that both hGDHs were very
rapidly imported and processed in isolated mitochondria, with their presequences
(N53) alone being capable of directing non-mitochondrial proteins into
mitochondria. These presequences were predicted to form two ? helices (?1: N
1-10; ?2: N 16-32) separated by loops. Selective deletion of the ?1 helix
abolished the mitochondrial import of hGDHs. While the ?1 helix alone had a very
weak hGDH mitochondrial import capacity, it could direct efficiently
non-mitochondrial proteins into mitochondria. In contrast, the ?2 helix had no
autonomous mitochondrial-targeting capacity. A peptide consisting of ?1 and ?2
helices without intervening sequences had GDH transport efficiency comparable
with that of N53. Mutagenesis of the cleavage site blocked the
intra-mitochondrial processing of hGDHs, but did not affect their mitochondrial
import. Replacement of all three positively charged N-terminal residues (Arg3,
Lys7 and Arg13) by Ala abolished import. We conclude that the synergistic
interaction of helices ?1 and ?2 is crucial for the highly efficient import of
hGDHs into mitochondria.