典型的色氨酸合酶是稳定的异四聚体α-β-α复合物。构成TrpA和TrpB1亚基,由trp操纵子中的相邻基因编码,以双向方式相互激活。最近,发现了一类新的TrpB2蛋白,其成员含有可能在空间上阻止与TrpA形成复合物的额外氨基酸。为了检验这个假设,我们对sulfolobussolfataricus的TrpA和TrpB蛋白进行了表征。这种嗜高温古细菌不包含TrpB1蛋白,而是包含两个在trp操纵子(TrpB2i)和trp操纵子(TrpB2o)外部编码的TrpB2同源物。我们发现TrpB2i和TrpA在催化过程中形成弱的瞬时络合物,通过TrpB2i单向激活TrpA。相比之下,TrpB2o和TrpA不形成可检测的复合物。这些结果表明了色氨酸合酶进化的模型,其中TrpB2o,TrpB2i,和TrpB1反映了TrpB对TrpA亲和力的逐步增加和功能亚基相互作用的完善,伴随着trp操纵子中编码基因的共定位。
The prototypical tryptophan synthase is a stable heterotetrameric alpha-betabeta-alpha complex. The constituting TrpA and TrpB1 subunits, which are encoded by neighboring genes in the trp operon, activate each other in a bi-directional manner. Recently, a novel class of TrpB2 proteins has been identified, whose members contain additional amino acids that might sterically prevent complex formation with TrpA. To test this hypothesis, we characterized the TrpA and TrpB proteins from Sulfolobus solfataricus. This hyperthermophilic archaeon does not contain a TrpB1 protein but instead contains two TrpB2 homologues that are encoded within (TrpB2i) and outside (TrpB2o) the trp operon. We find that TrpB2i and TrpA form a weak and transient complex during catalysis, with a uni-directional activation of TrpA by TrpB2i. In contrast, TrpB2o and TrpA do not form a detectable complex. These results suggest a model for the evolution of the tryptophan synthase in which TrpB2o, TrpB2i, and TrpB1 reflect the stepwise increase of TrpB affinity for TrpA and the refinement of functional subunit interaction, concomitant with the co-localization of the encoding genes in the trp operon.