Protein secretion prokaryotic

Thus, signal sequences are not gene-specific they can direct translocation of other secretory proteins. Furthermore, the secretion apparatus in one type of cell can recognize signal sequences from other types of cell, and secrete the attached protein. That prokaryotic proteins can be secreted in eukaryotic cells and vice versa indicates that the secretion apparatus has changed little (at least in some respects) in the process of evolution. 

Information on components of the prokaryotic protein-secretion apparatus has come primarily from genetic studies in E. coli. The chief technique has been to obtain a strain that is defective in some aspect of protein secretion and then to screen for strains in which the defect is suppressed by a compensating second-site mutation (Emr et al., 1981 Bankaitis and Bassford, 1985). Another approach has been to select for mutations that exhibit a pleiotropic export-defective phenotype (Ito et al., 1983 Oliver and Beckwith, 1981). These procedures have identified a host of genetic loci that specify proteins that affect secretion. Two of... 

Wickner (1980) proposed an alternative mechanism of protein secretion, called the membrane trigger hypothesis. This model proposes that the signal sequence influences the precursor protein or a domain of the precursor to fold into a conformation that can spontaneously partition into the hydrophobic part of the bilayer. In prokaryotes, the membrane potential causes the protein to traverse the bilayer. The protein then regains a water-soluble conformation, and is expelled into the medium. Signal peptidase removes the signal sequence during or after this process. Thus, secretory proteins or domains are transported across the membrane posttranslationally without the aid of a proteinaceous secretory apparatus. An energy source, such as the membrane potential, is required for secretion. 

Figure 27.30 A current model for protein secretion by prokaryotes. 

Furthermore, other bacteria have been shown to have superior innate abilities to secrete proteins including Bacillus subtilis, a Gram-positive bacterium, which lacks membrane proteases. This indicates that prokaryotes could... 

In addition to the destructive proteolysis processes in the proteasome and lysosome, many constructive proteolysis processes occur in cells. In both prokaryotes and eukaryotes, secreted proteins contain a signal peptide at the N-terminus that directs them to the secretary pathway. This signal peptide must be cleaved later by signal peptidases (typically serine proteases)... 

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