A REGIONAL NET CHARGE AND STRUCTURAL COMPENSATION MODEL TO EXPLAIN HOW NEGATIVELY CHARGED AMINO-ACIDS CAN BE ACCEPTED WITHIN A MITOCHONDRIAL LEADER SEQUENCE

Mitochondrial leader sequences have been found to be statistically enr iched for positively charged residues, with only a few known leader se quences possessing negatively charged residues. Mutational studies tha t have introduced negatively charged residues into various leader sequ ences have shown a general, but not absolute, trend toward reduced imp ort. 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, p rovided that a net positive charge remained in the N-helical segment. When the net charge of the N-terminal helical segment was reduced to z ero, import could be recovered by removing the linker, which resulted in a longer, more stable leader. This structural recovery of import wa s effective enough to compensate for a net charge of zero within the f irst 10 residues, even when a glutamate is the first charged side chai n presented in the sequence.