Proteins destined for translocation across the prokaryotic cytoplasmic membrane are synthesized as precursors carrying transient N-terminal extensions known as signal sequences. They facilitate initial engagement of precursor proteins with the sec-dependent translocase to initiate active threading of the polypeptide across the membrane. The translocated precursor is then processed by a transcytoplasmic signal peptidase anchored to the inner membrane. The temporal nature of cleavage of the signal sequence during pre-protein translocation has remained elusive. Using an engineered mammalian cytochrome b(5) precursor we demonstrate that the signal peptide processing in Escherichia coli is an event that can occur after almost complete exocytoplasmic translocation of the preprotein is accomplished. We discuss implications of the findings in light of the known working model of sec-dependent pre-protein translocon.

中文翻译：

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信号肽酶I介导的工程化哺乳动物细胞色素b（5）前体的处理是大肠埃希氏菌中的胞浆外易位事件。

预定用于跨原核细胞质膜易位的蛋白质被合成为携带瞬时N端延伸的前体，称为信号序列。它们促进了前体蛋白与sec依赖性转位酶的初始结合，从而启动了多肽跨膜的主动穿线。然后通过锚定在内膜上的胞质信号肽酶处理易位的前体。蛋白质前转运过程中信号序列裂解的时间性质仍然难以捉摸。使用工程化的哺乳动物细胞色素b（5）前体，我们证明了在大肠埃希氏菌中信号肽加工是一个事件，该事件可能发生在预蛋白的几乎完全胞质易位完成之后。