Jaussi R
Institute of Medical Radiobiology, University of Zürich, Switzerland.
Eur J Biochem. 1995 Mar 15;228(3):551-61. doi: 10.1111/j.1432-1033.1995.tb20294.x.
Mitochondrial and cytosolic proteins may be expected to differ in specific traits due to their different intracellular location. However, the identification of these differences between mitochondrial and cytosolic proteins is complicated by the heterogeneity of the two protein groups. These difficulties have been overcome by comparing traits of homologous genes, which are derived from a common ancestor gene, and their gene products. An earlier report [Hartmann, C., Christen, P. & Jaussi, R. (1991) Nature 352, 762-763] describing a positive net charge difference between the mature parts of nuclear-encoded mitochondrial proteins and their homologous cytosolic isoproteins, could be corroborated by extending the data collection. New data were gathered from computer databases and published studies. The average isoelectric points of the mitochondrial and cytosolic isoproteins are 7.5 and 6.5, respectively. Depending on the type of protein, the observed difference results from differences in the number of basic and/or acidic amino acid residues in the isoproteins. Probably both the conditions required for mitochondrial protein import and the local conditions within the organelle furthered the evolution of basic protein structures. The contribution of the mitochondrial targeting peptide to the positive charge of precursors of nuclear-encoded mitochondrial proteins is largest when the value of the isoelectric point of the mature protein is small. This mutual dependence of the charge of the targeting peptide and the mature protein part supports the notion that positive charge is essential for mitochondrial protein import. Several traits other than electric charge, i.e. codon usage, chromosome location, structural organization or regulation of the genes, do not show specific differences between the sets of the heterotopic isoproteins. There is no preference of gene location for any of the gene sets; only rarely are the genes for a mitochondrial and a cytosolic isoprotein located on the same chromosome. A variant of the 3' splice-site consensus exists in genes of nuclear-encoded mitochondrial proteins. This is most likely a consequence of the evolution of the genes in separate lineages before endosymbiosis led to the formation of mitochondria. Some of the original mRNA group II intron self-splicing functions of the endosymbiont seem to persist in part of the cytosolic splicing machinery and apparently require a specific consensus sequence [Juretic, N., Jaussi, R., Mattes, U. & Christen, P. (1987) Nucleic Acids Res.15, 10083-10086].
由于线粒体蛋白和胞质蛋白在细胞内的位置不同，它们可能在特定特征上存在差异。然而，由于这两类蛋白质的异质性，鉴定线粒体蛋白和胞质蛋白之间的这些差异变得复杂。通过比较源自共同祖先基因的同源基因及其基因产物的特征，克服了这些困难。一份早期报告[Hartmann, C., Christen, P. & Jaussi, R. (1991) Nature 352, 762 - 763]描述了核编码线粒体蛋白的成熟部分与其同源胞质同工蛋白之间存在正净电荷差异，通过扩展数据收集可以证实这一点。新数据来自计算机数据库和已发表的研究。线粒体和胞质同工蛋白的平均等电点分别为7.5和6.5。根据蛋白质的类型，观察到的差异是由于同工蛋白中碱性和/或酸性氨基酸残基数量的差异。线粒体蛋白导入所需的条件以及细胞器内的局部条件可能都促进了碱性蛋白质结构的进化。当成熟蛋白的等电点值较小时，线粒体靶向肽对核编码线粒体蛋白前体正电荷的贡献最大。靶向肽电荷与成熟蛋白部分的这种相互依赖性支持了正电荷对线粒体蛋白导入至关重要的观点。除电荷外的其他几个特征，即密码子使用、染色体位置、基因的结构组织或调控，在异位同工蛋白组之间没有显示出特定差异。对于任何一组基因，都没有基因位置偏好；线粒体和胞质同工蛋白的基因很少位于同一条染色体上。核编码线粒体蛋白的基因中存在3'剪接位点共有序列的变体。这很可能是在共生导致线粒体形成之前，基因在不同谱系中进化的结果。内共生体的一些原始mRNA II组内含子自我剪接功能似乎部分保留在胞质剪接机制中，并且显然需要特定的共有序列[Juretic, N., Jaussi, R., Mattes, U. & Christen, P. (1987) Nucleic Acids Res.15, 10083 - 10086]。
