Protein Targeting and Degradation: -Bacteria Also Use Signal Sequences for Protein Targeting
Bacteria can target proteins to their inner or outer mem branes, to the periplasmic space between these mem branes, or to the extracellular medium. They use signal sequences at the amino terminus of the proteins (Fig.2), much like those on eukaryotic proteins targeted to the ER, mitochondria, and chloroplasts. Most proteins exported from E. coli make use of the pathway shown in Figure 3. Following translation, a protein to be exported may fold only slowly, the amino-terminal signal sequence impeding the folding. The soluble chaperone protein SecB binds to the protein’s signal sequence or other features of its incompletely folded structure. The bound protein is then de livered to SecA, a protein associated with the inner surface of the plasma membrane. SecA acts as both a receptor and a translocating ATPase. Released from SecB and bound to SecA, the protein is delivered to a translocation complex in the membrane, made up of SecY, E, and G, and is translocated stepwise through the membrane at the SecYEG complex in lengths of about 20 amino acid residues. Each step is facilitated by the hydrolysis of ATP, catalyzed by SecA.
FIGURE 1 Targeting of nuclear proteins. (a) 1 A protein with an appropriate nuclear localization signal (NLS) is bound by a complex of importin and. 2 The resulting complex binds to a nuclear pore, and 3 translocation is mediated by the Ran GTPase. 4 Inside the nucleus, importin dissociates from importin, and 5importin then releases the nuclear protein. 6 Importin and are transported out of the nucleus and recycled. (b)Scanning electron micrograph of the surface of the nuclear envelope, showing numerous nuclear pores.
FIGURE 2 Signal sequences that target proteins to different lo cations in bacteria. Basic amino acids (blue) near the amino terminus and hydrophobic core amino acids (yellow) are highlighted. The cleavage sites marking the ends of the signal sequences are indicated by red arrows. Note that the inner bacterial cell membrane (see Fig. 1–6) is where phage fd coat proteins and DNA are assembled into phage particles. OmpA is outer membrane protein A; LamB is a cell surface receptor protein for bacteriophage lambda.
FIGURE 3 Model for protein export in bacteria. 1 A newly translated polypeptide binds to the cytosolic chaperone protein SecB, which 2 delivers it to SecA, a protein associated with the translocation complex (SecYEG) in the bacterial cell membrane. 3 SecB is released, and SecA inserts itself into the membrane, forcing about 20 amino acid residues of the protein to be exported through the translocation complex. 4 Hydrolysis of an ATP by SecA provides the energy for a conformational change that causes SecA to withdraw from the membrane, releasing the polypeptide. 5 SecA binds another ATP, and the next stretch of 20 amino acid residues is pushed across the membrane through the translocation complex. Steps 4 and 5 are repeated until 6 the entire protein has passed through and is released to the periplasm. The electrochemical potential across the membrane (denoted by + and -) also provides some of the driving force required for protein translocation.
An exported protein is thus pushed through the membrane by a SecA protein located on the cytoplasmic surface, rather than being pulled through the mem brane by a protein on the periplasmic surface. This difference may simply reflect the need for the translocating ATPase to be where the ATP is. The transmembrane electrochemical potential can also provide energy for translocation of the protein, by an as yet unknown mechanism. Although most exported bacterial proteins use this pathway, some follow an alternative pathway that uses signal recognition and receptor proteins homologous to components of the eukaryotic SRP and SRP receptor .
