| 其他摘要 | Ras superfamily proteins are a group of ubiquitous small GTP binding proteins in eukaryotes. They share a high conserved GTP binding domain and have been classified into seven familes: Sar1, Arf, SRβ, Ran, Rab, Rho and Ras, based on their sequence characters and cellular functions. Each of these proteins performs specific cellular functions such as the vesicle trafficking (Sar1, Arf and Rab)，the cytoskeleton remodeling (Rho), the nucleocytoplasmic transport and the nuclear envelope remodeling (Ran) etc. The origin and evolution of the Ras superfamily proteins are closely related to the origin and evolution of the eukaryotes. By using the methods of bioinfamatics and through the experiments of molecular biology, the distribution of the Ras superfamily proteins in prokaryotes and protists were investigated, and further molecular phylogenetic analyses were performed. The origin and evolution of the Ras superfamily proteins were studied systematically and thoroughly. The results and conclusions are as follows: 1) Through searching the prokaryotic genomic databases and analyzing the sequence characters, the prokaryotic sequences showing high similarities to the eukaryotic Ras superfamily proteins were firstly identified in eubacteria，and the experiments indicated that the genes of them were actively transcribed. In Archaea, some sequences showing relatively high divergence from the eukaryotic Ras superfamily proteins were found in methanogenic ones and Thermoplasmales. The MglA proteins, which have been recognized as the prokaryotic small GTP binding proteins，were found in more eubacteria. Sequence alignment shows that MglA proteins have their own unique features very distinct from the eukaryotic Ras superfamily proteins. Further molecular phylogenetic analysis shows that among the seven families of the eukaryotic Ras superfamily proteins, four families (Ran, Rab, Rho and Ras) cluster together with the eubacterial ones showing high similarities to them closely branching outside of them; the other three families (Sar1, Arf and SRβ) form an another separate branch, clustering with the methanogenic archaebacterial homologues and eubacterial MglA proteins. These results suggests that the Ras superfamily proteins are not specific for eukaryotes, which indeed have originated in some prokaryotes; the ancestors of eukaryotic Ras superfamily proteins also should not be the eubacterial MglA as the people thoughted before; the seven families of eukaryotic Ras superfamily proteins may have two different origins: Ran, Rab, Rho and Ras evolve from Cyanobacteria, or alpha-proteobacteria, or the common ancestor of them, while Sar1, Arf, and Srbeta derive from methanogenic archaea, which may reflect the “fusion” evolutionary history of the eukaryotic cells. 2) Through searching the genomic databases of some relatively low unicellular eukaryotes, protists, and performing systematically analyses by combining the search results with the Ras superfamily proteins in a series of other eukaryotes in different evolutionary status, the Sar1, Arf, Rab and Ran were found to be ubiquitous in eukaryotes, while SRβ, Rho and Ras were not found in some eukaryotes. Based on the distribution conditions of each family members in eukaryotes, it is proposed that the composition of Ras superfamily proteins in the last common ancestor of eukaryotes may have two conditions: (1) the last common ancestor of eukaryotes have had all seven families mermers, and at least have eleven members: 1 Sar1, 1 SRβ，3 Arf (Arf1, Arl1, Arl2), 3 Rab (Rab1, Rab6, Rab11), 1 Ran, 1 Rho (Rac), 1 Ras (RheB). Therefore, the lack of SRβ, Rho and Ras families members in some eukaryotes probably is due to the gene losses. The absence of Ras family members in plants should be attributed to the loss of single Ras family protein gene in their ancestor of green algae; (2) based on Cavalier-Smith’ s classification of the euakryotes, in which the eukaryotes are classified into two big clades (the unikonts (Amoebazoa, fungi and metazoa) and bikonts (algaes, plants and the protozoa except Amoebazoa)), the last common ancestor of eukaryotes may only have the six family members except the Ras family, and the Ras family membes may derive in unikonts after the two big groups have diverged. The lack of Ras family members in green algae, plants and some protozoa may be an ancestral character. And the Ras family members in some bikonts such as the red algae may have been obtained through the lateral gene transfer from the unikonts. As for the absence of SRβ and Rho, it still may be due to the gene loss. In addition, the very large Ras superfamily proteins in Amoebazoa organisms suggested that the genome size and the evolutionary status are not the deciding factor of the small or large of the Ras superfamily proteins, and the corresponding requirements of cellular physiologic activies are indeed the key reason of the family members increasing. |
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