It is becoming increasingly clear that similarities exist in the manner in which extracytoplasmic proteins are targeted to complexes responsible for translocating these proteins across membranes in each of the three domains of life. In Eukarya and Bacteria, the signal recognition particle (SRP) directs nascent polypeptides to membrane-embedded translocation sites. In Archaea, the SRP protein targeting pathway apparently represents an intermediate between the bacterial and eukaryal systems. Understanding the archaeal SRP pathway could therefore reveal universal aspects of targeting not detected in current comparisons of the eukaryal and bacterial systems while possibly identifying aspects of the process either not previously reported or unique to Archaea.
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