Mehner-Breitfeld Denise, Schwarzkopf Jan Michel Frederik, Young Ry, Kondabagil Kiran, Brüser Thomas
Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany.
Department of Biochemistry & Biophysics, Center of Phage Technology, Texas A&M University, College Station, TX, United States.
Front Microbiol. 2021 Aug 11;12:712460. doi: 10.3389/fmicb.2021.712460. eCollection 2021.
Holin/endolysin-mediated lysis of phage T4 of is tightly regulated by the antiholins RI and RIII. While regulation by the cytoplasmic RIII plays a minor role, the periplasmic antiholin RI binds tightly to the holin T and is believed to directly sense periplasmic phage DNA from superinfections as a trigger for the inhibition of lysis. RI has been reported to contain a non-cleavable signal peptide that anchors the protein to the membrane. Lysis is believed to be induced at some stage by a membrane depolarization that causes a release of RI into the periplasm without cleavage of the signal anchor. For the current model of phage lysis induction, it is thus a fundamental assumption that the N-terminal trans-membrane domain (TMD) of RI is such a signal anchor release (SAR) domain. Here we show that, in contrast to previous reports, this domain of RI is a cleavable signal peptide. RI is processed and released into the periplasm as a mature protein, and inactivation of its signal peptidase cleavage site blocks processing and membrane release. The signal peptide of RI can also mediate the normal translocation of a well-characterized Sec substrate, PhoA, into the periplasm. This simplifies the current view of phage lysis regulation and suggests a fundamentally different interpretation of the recently published structure of the soluble domains of the RI-T complex.
噬菌体T4的溶菌酶/内溶素介导的裂解受抗溶菌酶RI和RIII的严格调控。虽然细胞质中的RIII起的调控作用较小，但周质抗溶菌酶RI与溶菌酶T紧密结合，据信它能直接感知来自超感染的周质噬菌体DNA，作为抑制裂解的触发因素。据报道，RI含有一个不可裂解的信号肽，该信号肽将蛋白质锚定在膜上。据信，在某个阶段，膜去极化会导致RI释放到周质中而信号锚不被裂解，从而诱导裂解。因此，对于当前的噬菌体裂解诱导模型来说，一个基本假设是RI的N端跨膜结构域（TMD）是这样一个信号锚释放（SAR）结构域。在这里，我们表明，与之前的报道相反，RI的这个结构域是一个可裂解的信号肽。RI被加工并作为成熟蛋白释放到周质中，其信号肽酶切割位点的失活会阻止加工和膜释放。RI的信号肽还可以介导一个特征明确的Sec底物PhoA正常转运到周质中。这简化了当前对噬菌体裂解调控的认识，并对最近发表的RI-T复合物可溶性结构域的结构提出了根本不同的解释。
