Apocytochrome c (Apo c), heme-free cytochrome c, is synthesized in the cell cytoplasm without a cleaved signal sequence, and then translocated across the outer mitochondrial membrane. The interaction of Apo c with phospholipid vesicles has been studied as a model for understanding protein translocation across membrane. The translocation of Apo c across model membrane has been studied by using large unilamellar, trypsin-containing vesicles at neutral pH. It is observed with the electrophoretic experiment that the amount of Apo c decreases and that of a certain segment increase with time. The size of the segment is about 2.5 kDa, and larger than any segments that can be produced by complete tryptic digestion of Apo c. Also, the segment is labeled with hydrophobic probe partitioned in phospholipid bilayer. Thus, a part of Apo c is exposed to the internal trypsin of the vesicles and some segment is protected against tryptic digestion by phospholipid bilayer. The N-terminal amino acid of 2.5 kDa segment was proved to be methionine with dansylation method. So, the segment is the C-terminal part of Apo c. These results provide us the clue to understand the mechanism of Apo c-translocation.
Apo c 가 미토콘드리아의 외막을 통과하는 mechanism을 이해하기 위해서 모델 시스템을 사용해서 실험을 수행하였다. 이 실험에서는 미토콘드리아 대신에 trypsin을 내재시킨 PS 막 소포를 사용하였다. 중성 pH에서 Apo c 일부가 이 막 소포를 통과하여 가수분해됨이 확인되었다. 또한, 가수분해되지 않고 남은 부분의 크기는 2.5 kDa이었으며, 이것은 α-helix 형태로 막을 한번 횡단하는 크기다. Dansylation 방법으로 이 부분이 Apo c 의 C-말단부위임이 밝혀졌다. 이런 결과들은 Apo c 가 PS 막 소포를 hairpin loop 형태로 통과함을 강력히 시사한다.
