Figure 225

Summary of events during protein synthesis. Prior to the events depicted here, DNA has been "translated" into mRNA, and the large and small ribosomal subunits have assembled into a ribosome. Notice that the ribosome has two binding sites: the "P" site and the "A" site. 1. Transfer RNA (tRNA) with a growing peptide chain is bound to the P site of the ribosome. The initial amino acids of the nascent protein constitute the signal sequence (red). The nascent protein grows by adding amino acids at the end of the chain opposite to the signal sequence. 2. An incoming tRNA with an amino acid attached binds to the A site. 3. A peptide bond forms between the last amino acid of the growing peptide chain and the new amino acid brought to the ribosome by the incoming tRNA. 4. The previous tRNA is released from the P site. 5. The "new" tRNA with the growing peptide chain is translocated to the A site, and the ribosome moves along the mRNA. 6. The ribosome is attached to the membrane of the rER, and after recognizing the membrane pore proteins, the signal sequence becomes translocated into the lumen of the rER. 7. By enzymatic action of a signal peptidase, signal sequence is cleaved from the peptide. 8. After the stop codon is recognized by the ribosome, the synthesis is terminated, and both of the ribosomal sub-units dissociate from the mRNA and the surface of the rER.

present there; these modifications include core glycosyla-tion, disulfide and internal hydrogen bond formation, folding of the newly synthesized protein, and partial subunit assembly. The rER also serves as a quality checkpoint in the process of protein production. If the newly synthesized protein is not properly modified, it cannot exit the rER.

Except for the few proteins that remain permanent residents of the rER membranes and those proteins secreted by the constitutive pathway, the newly synthesized proteins are normally delivered to the Golgi apparatus within minutes. In cells in which the constitutive pathway is dominant, namely, plasma cells and developing fibroblasts, newly synthesized proteins may accumulate in the rER cis-ternae, causing their engorgement and distension.

Coatomers mediate bidirectional traffic between the rER and Golgi apparatus

Experimental data indicate that two classes of coated vesicles are involved in the transport of protein from and to the rER. A protein coat similar to clathrin surrounds vesicles transporting proteins between the rER and the Golgi apparatus (page 25). However, unlike clathrins, which mediate bidirectional transport from and to the plasma membrane, one class of proteins is involved only in anterograde transport from the rER to the as-Golgi network (CGN), the Golgi cisternae closest to the rER. Another class of proteins mediates retrograde transport from the CGN back to the rER (Fig. 2.26). These two classes of proteins are called coatomers or COPs:

• COP-I mediates transport vesicles originating in the CGN back to the rER (Fig. 2.27a). This retrograde transport mediates a salvage operation that returns rER proteins mistakenly transferred to the CGN during normal anterograde transport.

0 0

Post a comment