A regional net charge and structural compensation model to explain how negatively charged amino acids can be accepted...

Mitochondrial leader sequences have been found to be statistically enriched for positively charged residues, with only a few known leader sequences possessing negatively charged residues. Mutational studies that have introduced negatively charged residues into various leader sequences have shown a general, but not absolute, trend toward reduced import. The leader sequence of rat liver aldehyde dehydrogenase has been previously determined by NMR to form a helix-linker-helix structure. A negative charge introduced into this leader did not prevent import, provided that a net positive charge remained in the N-helical segment. When the net charge of the N-terminal helical segment was reduced to zero, import could be recovered by removing the linker, which resulted in a longer, more stable leader. This structural recovery of import was effective enough to compensate for a net charge of zero within the first 10 residues, even when a glutamate is the first charged side chain presented in the sequence.