They can also be released into the extracellular environment or directly translocated into host cells . All protein synthesis takes place in the cytoplasm, so all non-cytoplasmic proteins must pass through one or two lipid bilayers by a mechanism commonly called “”secretion”". Protein secretion is involved in various processes including plant-microbe interactions [4, 5]), biofilm formation
[6, selleck screening library 7] and virulence of plant and human pathogens [8–10]. Two main systems are involved in protein translocation across the cytoplasmic membrane, namely the essential and universal Sec (Secretion) pathway and the Tat (Twin-arginine translocation) pathway found in some prokaryotes (monoderms and diderms) and eukaryotes alike [11–16]. The Sec machinery recognizes an N-terminal hydrophobic signal sequence and translocates unfolded proteins , whereas the Tat machinery recognizes a basic-rich N-terminal motif (SRR-x-FLK) and transports fully folded proteins [13, 14]). In addition to these systems, diderm bacteria have six further systems that secrete proteins using a contiguous channel spanning the two membranes (T1SS, [17, 18], T3SS, T4SS and T6SS [19–24]) or in two steps, the first being Sec- or Tat-dependent
export into the periplasmic and the second being translocation across the outer membrane (T2SS, [25–27] and T5SS, [28, 29]). Other diderm protein secretion systems exist: they include the chaperone-usher system (CU or T7SS, tuclazepam [30, 31]) and the
extracellular nucleation-precipitation mechanism (ENP or T8SS, ). It is Proteasome inhibition assay worth mentioning that the terminology T7SS has also been proposed to describe a completely different protein secretion system, namely the ESAT-6 protein secretion (ESX) in Mycobacteria, now considered as diderm bacteria . Beside Sec and Tat pathways, monoderm bacteria have additional secretion systems for protein translocation across the cytoplasmic membrane, namely the flagella export apparatus (FEA ), the fimbrilin-protein exporter (FPE, [35, 36]) and the WXG100 secretion system (Wss, [37, 38]). Establishing whole proteome subcellular localization by biochemical experiments is possible but arduous, time consuming and expensive. Data concerning predicted proteins (from whole genome sequences) is continuously increasing. High-throughput in silico analysis is required for fast and accurate prediction of additional attributes based solely on their amino acid sequences. There are large numbers of global (that yield final localization) and specialized (that predict features) tools for computer-assisted prediction of protein localizations. Most specialized tools tend to detect the presence of N-terminal signal peptides (SP). Prediction of Sec-sorting signals has a long history as the first methods, based on weight matrices, were published about fifteen years ago [39–41]. Numerous machine learning-based methods are now available [42–50].