Doolittle R F
Center for Molecular Genetics, University of California, San Diego, La Jolla 92093-0634, USA.
Philos Trans R Soc Lond B Biol Sci. 1995 Sep 29;349(1329):235-40. doi: 10.1098/rstb.1995.0107.
The common ancestry of eukaryotes, archaebacteria and eubacteria is well demonstrated by amino acid sequence comparisons of numerous proteins that are common to all three groups. On the other hand, there are a few proteins, like ubiquitin, that are common to eukaryotes and archaebacteria and which have yet to be observed in eubacteria. Some proteins appear to be wholly restricted to eukaryotes; this is especially true of cytoskeletal proteins. Recently, actin has been found by crystallography to be homologous with an ATP-binding domain found in a heat shock protein and several other proteins common to all three urkingdoms. This observation is puzzling on several counts. Most cytoskeletal proteins like actin and tubulin are very slow changing and must have been so for a very long time. How is it, then, that no sequence resemblance can be discerned with their alledged prokaryotic antecedents? The question is addressed by considering two bacterial fts proteins which appear to be related to actin, on the one hand, and tubulin, on the other. One answer may be that the rate of change of these proteins changed dramatically at a key point in their history. Another possibility is that eukaryotes are much older than some of their other proteins indicate.
真核生物、古细菌和真细菌的共同祖先通过对这三类生物共有的众多蛋白质进行氨基酸序列比较得到了充分证明。另一方面，有一些蛋白质，如泛素，是真核生物和古细菌所共有的，而在真细菌中尚未观察到。有些蛋白质似乎完全局限于真核生物；细胞骨架蛋白尤其如此。最近，通过晶体学发现肌动蛋白与一种热休克蛋白以及所有三个超界共有的其他几种蛋白质中发现的ATP结合结构域同源。这一观察结果在几个方面令人困惑。大多数细胞骨架蛋白，如肌动蛋白和微管蛋白，变化非常缓慢，而且肯定已经如此很长时间了。那么，为什么与它们所谓的原核生物前身没有序列相似性呢？通过考虑两种细菌fts蛋白来解决这个问题，这两种蛋白一方面似乎与肌动蛋白有关，另一方面与微管蛋白有关。一个答案可能是这些蛋白质的变化速率在其历史的关键点上发生了巨大变化。另一种可能性是真核生物比它们的一些其他蛋白质所显示的要古老得多。
