Institute of Botany III, University of Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf, Germany.
Philos Trans R Soc Lond B Biol Sci. 2010 Mar 12;365(1541):847-55. doi: 10.1098/rstb.2009.0252.
Many genes in eukaryotes are acquisitions from the free-living antecedents of chloroplasts and mitochondria. But there is no evolutionary 'homing device' that automatically directs the protein product of a transferred gene back to the organelle of its provenance. Instead, the products of genes acquired from endosymbionts can explore all targeting possibilities within the cell. They often replace pre-existing host genes, or even whole pathways. But the transfer of an enzymatic pathway from one compartment to another poses severe problems: over evolutionary time, the enzymes of the pathway acquire their targeting signals for the new compartment individually, not in unison. Until the whole pathway is established in the new compartment, newly routed individual enzymes are useless, and their genes will be lost through mutation. Here it is suggested that pathways attain novel compartmentation variants via a 'minor mistargeting' mechanism. If protein targeting in eukaryotic cells possesses enough imperfection such that small amounts of entire pathways continuously enter novel compartments, selectable units of biochemical function would exist in new compartments, and the genes could become selected. Dual-targeting of proteins is indeed very common within eukaryotic cells, suggesting that targeting variation required for this minor mistargeting mechanism to operate exists in nature.
真核生物中的许多基因是从叶绿体和线粒体的自由生活祖先中获得的。但是，没有进化的“归巢装置”可以自动将转移基因的蛋白质产物引导回其起源的细胞器。相反，从内共生体获得的基因的产物可以在细胞内探索所有的靶向可能性。它们经常取代预先存在的宿主基因，甚至整个途径。但是，将酶途径从一个隔室转移到另一个隔室会带来严重的问题：在进化过程中，途径中的酶会单独获得针对新隔室的靶向信号，而不是同步获得。在新隔室中建立整个途径之前，新路由的单个酶是无用的，它们的基因将通过突变丢失。这里提出的途径通过“小靶向错误”机制获得新的隔室化变体。如果真核细胞中的蛋白质靶向具有足够的不完美性，使得小量的整个途径不断进入新隔室，则新隔室中会存在可选择的生化功能单元，并且这些基因可以被选择。蛋白质的双重靶向在真核细胞中确实非常普遍，这表明这种小靶向错误机制所需的靶向变化在自然界中存在。
