D. Michl et al., Phylogenetic transfer of organelle genes to the nucleus can lead to new mechanisms of protein integration into membranes, PLANT J, 17(1), 1999, pp. 31-40
Subunits CFo-I and CFo-II of ATP synthase in chloroplast thylakoid membrane
s are two structurally and functionally closely related proteins of bitopic
membrane topology which evolved from a common ancestral gene. In higher pl
ants, CFo-I still originates in plastid chromosomes (gene: atpF), while the
gene for CFo-II (atpG) was phylogenetically transferred to the nucleus. Th
is gene transfer was accompanied by the reorganization of the topogenic sig
nals and the mechanism of membrane insertion. CFo-I is capable of integrati
ng correctly as the mature protein into the thylakoid membrane, whereas mem
brane insertion of CFo-II strictly depends on a hydrophobic targeting signa
l in the transit peptide. This requirement is caused by three negatively ch
arged residues at the hi-terminus of mature CFo-II which are lacking from C
Fo-I and which have apparently been added to the protein only after gene tr
ansfer has occurred. Accordingly, the CFo-II transit peptide is structurall
y and functionally equivalent to typical bipartite transit peptides, capabl
e of also translocating hydrophilic lumenal proteins across the thylakoid m
embrane. In this case, transport takes place by the Sec-dependent pathway,
despite the fact that membrane integration of CFo-II is a Sec-independent,
and presumably spontaneous, process.