~EVIEWS
I n bacteria, there is a continuous exchange of molecules between the cytoplasm and the extracellular medium: nutrients are taken up and small molecules and macromolecules such as toxins or enzymes are secreted. The extracellular medium and intracellular compartment are separated by the bacterial envelope, which, in Gram-positive bacteria consists of the sole cytoplasmic membrane surrounded by the peptidoglycan cell wall, and in Gram-negative bacteria comprises a more differentiated structure, including the inner and outer membranes separated by a peptidoglycan layer and periplasmic space. The two membranes of Gram-negative bacteria contain many proteins, and they must be crossed by extracellular proteins during their secretion. These proteins are all synthesized on cytoplasmic polysomes but during or soon after translation they are translocated through one or both membranes to their final location. The mechanisms of protein translocation and targeting have been intensively studied during the past few years. In Escherichia coli, it has been establisl:led that protein export through the inner membrane to the periplasm or the outer membrane is achieved by a unique, apparently universal mechanism: the signal peptide-dependent general export pathway (GEP), which comprises the sec gene products and the signal peptidases (reviewed in Ref. 1). On the other hand, as described here, protein secretion through both membranes does not occur by a single mechanism but rather by multiple systems. It is possible to distinguish, very schematically, three different mechanisms of secretion (Fig. 1). (1) Most frequently, secretion is a two-step process: the secreted proteins cross the inner membrane to the periplasm by the GEP, then cross the outer membrane by a separate mechanism. This system has been named the general secretory pathway (GSP). (2) A few secreted proteins are exported across the inner membrane to the outer membrane by the GEP, then released from the outer membrane without the help of other proteins. (3) Finally, several secreted proteins cross both membranes in a single step, using a pathway radically different from the GEP, called the signal peptideindependent pathway.
The general secretory pathway (GSP) In this two-step pathway, proteins cross the inner membrane by the GEP and then the outer membrane with the help of specific secretion activities. The notion of a two-step process and of specific components first arose from the study of Erwinia chrysanthemi out (Ref. 2) and Pseudomonas aeruginosa xcp (Ref. 3) secretion-defective mutants, which accumulated in their periplasm several exoproteins, such as pectinases and cellulases (E. ch~santhemi 4) or exotoxin and elastase (P. aeruginosaS). These mutants had no phenotype other than the defect in protein secretion. Next, the cloning of the structural genes for some secreted proteins, such as pectinase or elastase, in the foreign host E. coli, led to the synthesis of the polypeptides, their translocatkm to the periplasm, but not their secretion
I n bacteria, there is a continuous exchange of molecules between the cytoplasm and the extracellular medium: nutrients are taken up and small molecules and macromolecules such as toxins or enzymes are secreted. The extracellular medium and intracellular compartment are separated by the bacterial envelope, which, in Gram-positive bacteria consists of the sole cytoplasmic membrane surrounded by the peptidoglycan cell wall, and in Gram-negative bacteria comprises a more differentiated structure, including the inner and outer membranes separated by a peptidoglycan layer and periplasmic space. The two membranes of Gram-negative bacteria contain many proteins, and they must be crossed by extracellular proteins during their secretion. These proteins are all synthesized on cytoplasmic polysomes but during or soon after translation they are translocated through one or both membranes to their final location. The mechanisms of protein translocation and targeting have been intensively studied during the past few years. In Escherichia coli, it has been establisl:led that protein export through the inner membrane to the periplasm or the outer membrane is achieved by a unique, apparently universal mechanism: the signal peptide-dependent general export pathway (GEP), which comprises the sec gene products and the signal peptidases (reviewed in Ref. 1). On the other hand, as described here, protein secretion through both membranes does not occur by a single mechanism but rather by multiple systems. It is possible to distinguish, very schematically, three different mechanisms of secretion (Fig. 1). (1) Most frequently, secretion is a two-step process: the secreted proteins cross the inner membrane to the periplasm by the GEP, then cross the outer membrane by a separate mechanism. This system has been named the general secretory pathway (GSP). (2) A few secreted proteins are exported across the inner membrane to the outer membrane by the GEP, then released from the outer membrane without the help of other proteins. (3) Finally, several secreted proteins cross both membranes in a single step, using a pathway radically different from the GEP, called the signal peptideindependent pathway.
The general secretory pathway (GSP) In this two-step pathway, proteins cross the inner membrane by the GEP and then the outer membrane with the help of specific secretion activities. The notion of a two-step process and of specific components first arose from the study of Erwinia chrysanthemi out (Ref. 2) and Pseudomonas aeruginosa xcp (Ref. 3) secretion-defective mutants, which accumulated in their periplasm several exoproteins, such as pectinases and cellulases (E. ch~santhemi 4) or exotoxin and elastase (P. aeruginosaS). These mutants had no phenotype other than the defect in protein secretion. Next, the cloning of the structural genes for some secreted proteins, such as pectinase or elastase, in the foreign host E. coli, led to the synthesis of the polypeptides, their translocatkm to the periplasm, but not their secretion
