Rankin S E, Watts A, Pinheiro T J
Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K.
Biochemistry. 1998 Sep 8;37(36):12588-95. doi: 10.1021/bi980408x.
In aqueous solution, while cytochrome c is a stably folded protein with a tightly packed structure at the secondary and tertiary levels, its heme-free precursor, apocytochrome c, shows all features of a structureless random coil. However, upon interaction with phospholipid vesicles or lysophospholipid micelles, apocytochrome c undergoes a conformational transition from its random coil in solution to an alpha-helical structure on association with lipid. The driving forces of this lipid-induced folding process of apocytochrome c were investigated for the interaction with various phospholipids and lysophospholipids. Binding of apocytochrome c to negatively charged phospholipid vesicles induced a partially folded state with approximately 85% of the alpha-helical structure of cytochrome c in solution. In contrast, in the presence of zwitterionic phospholipid vesicles, apocytochrome c remains a random coil, suggesting that negatively charged phospholipid headgroups play an important role in the mechanism of lipid-induced folding of apocytochrome c. However, negatively charged lysophospholipid micelles induce a higher content of alpha-helical structure than equivalent negatively charged diacylphospholipids in bilayers, reaching 100% of the alpha-helix content of cytochrome c in solution. Furthermore, micelles of lysolipids with the same zwitterionic headgroup of phospholipid bilayer vesicles induce approximately 60% of the alpha-helix content of cytochrome c in solution. On the basis of these results, we propose a mechanism for the folding of apocytochrome c induced by the interaction with lipid, which accounts for both electrostatic and hydrophobic contributions. Electrostatic lipid-protein interactions appear to direct the polypeptide to the micelle or vesicle surface and to induce an early partially folded state on the membrane surface. Hydrophobic interactions between nonpolar residues in the protein and the hydrophobic core of the lipid bilayer stabilize and extend the secondary structure upon membrane insertion.
在水溶液中，细胞色素c是一种结构稳定的折叠蛋白，在二级和三级水平上具有紧密堆积的结构，而其无血红素前体脱辅基细胞色素c则呈现出无结构的随机卷曲的所有特征。然而，当与磷脂囊泡或溶血磷脂胶束相互作用时，脱辅基细胞色素c会经历构象转变，从溶液中的随机卷曲转变为与脂质结合时的α螺旋结构。研究了脱辅基细胞色素c这种脂质诱导折叠过程的驱动力，涉及与各种磷脂和溶血磷脂的相互作用。脱辅基细胞色素c与带负电荷的磷脂囊泡结合会诱导出一种部分折叠状态，溶液中约85%的细胞色素c呈现α螺旋结构。相比之下，在两性离子磷脂囊泡存在的情况下，脱辅基细胞色素c仍保持随机卷曲状态，这表明带负电荷的磷脂头部基团在脱辅基细胞色素c脂质诱导折叠机制中起重要作用。然而，带负电荷的溶血磷脂胶束比双层中同等带负电荷的二酰基磷脂诱导出更高含量的α螺旋结构，达到溶液中细胞色素cα螺旋含量的100%。此外，具有与磷脂双层囊泡相同两性离子头部基团的溶血磷脂胶束诱导出溶液中约60%的细胞色素cα螺旋含量。基于这些结果，我们提出了一种脱辅基细胞色素c与脂质相互作用诱导折叠的机制，该机制考虑了静电和疏水作用。静电脂质 - 蛋白质相互作用似乎将多肽引导至胶束或囊泡表面，并在膜表面诱导出早期的部分折叠状态。蛋白质中非极性残基与脂质双层疏水核心之间的疏水相互作用在膜插入时稳定并扩展二级结构。
