Gamma amino butyric acid (GABA) is used as drugs, food ingredients, and dietary supplements. l-glutamate is converted to GABA by the decarboxylation reaction, which is catalyzed by the glutamate decarboxylase (GAD). Escherichia coli is widely being used to express proteins. However, without appropriate signal peptide, it cannot be applied for secretory proteins. Selecting a suitable signal peptide (SP) is a critical step in the secretory production of different proteins. In silico identification of suitable SP is a reliable and cost-effective alternative to experimental approaches. In previous studies, the localization of proteins was not considered and the SPs of periplasmic, membranes and extracellular were compared. Therefore, this study aimed to predict the best SP for the expression of recombinant GAD in the outer membrane of E. coli only. Also, we compared twelve servers to evaluate protein localization, solubility, and secretory pathway. In the present study, 127 SPs were taken from the Signal Peptide database. The localization site, physico-chemical properties, location of cleavage sites, regions and D-score of them were determined by ProtComp, ProtParam, and SignalP 3.0 and 4.1 servers, respectively. To rank SPs based on the secretion properties, PRED-TAT and SignalP 5.0 webservers were used. Based on the results, the localization site of 13 SPs was in the outer membrane of E. coli. Among them, the most suitable candidates seemed to be torT with a reasonably high D-score, aliphatic index, and GRAVY, followed by ccmH and then pspE. TorT accelerates GAD scale-up production and might be useful in future experimental research.

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