Mechanism of Protein Synthesis (Explained With Diagram) ..
Figure 5. Sorting of proteins between the ER and GC. (a) Cargo proteins destined for secretion and v-SNAREs needed for carrier vesicle fusion to the GC are sorted away from the ER resident proteins into COPII coated vesicles. After delivery to the GC, the cargo proteins continue through the secretory pathway. (b) However, v-SNARES needed for the ER to GC trafficking step and ER resident proteins that have been mistakenly transported to the GC can be retrieved from the secretory pathway through retrograde transport back to the ER by COPI-coated carrier vesicle. ERGIC-53, a likely carrier protein, cycles between the ER and GC and may assist in sorting the secretory proteins from proteins that are retained in the ER.
leading to premature termination of protein synthesis.
Translocation of newly synthesized proteins across the ER membrane shows many similarities to translocation across the plasma membrane protein of bacteria (1, 15, 16). Proteins are prevented from folding in the cytoplasm. They are fed across the plasma membrane through a translocon, a proteinaceous pore, which has three subunits very similar to the bacterial proteins made by the secY, E, and G genes. By electron microscopy, these pores are rings about 8 to 10 nm in diameter, with a central pore of 2 nm, sufficient to allow the passage of an extended, hydrated peptide of 1.1 nm in diameter. These pores can now be recognized (17). In yeast, proteins traverse pores in the ER by two different types of translocation mechanisms. One is an ATP-driven process that translocates proteins whose synthesis is complete. The other couples translation to the translocation process. In this transport mode, the ribosome is attached to the proteinaceous transport pore, the translocon, and feeds the nascent train through the pore as it is being synthesized. Mammalian cells only have the co-translation made of translocation. When translocation is co-translational, the nascent chain is recognized in the cytoplasm by a signal recognition particle, which stops further protein synthesis until the complex of ribosome, nascent chain, and signal recognition particle reaches the endoplasmic reticulum (Fig. 3).
Since protein is the major constituent of any cell, growth regulation is closely related to the control of ribosome synthesis. In fact, the number of ribosomes per. Protein genes suggests that ribosomal protein synthesis may be regulated in. 1980, to control for differential loading of RNA on each gel lane. Because the. Ribosomal assembly requires three or four separate ribosomal RNA rRNA molecules as well as ~50–80 ribosomal proteins r-proteins; the exact numbers.
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To take correctly folded and oligomerized proteins from the ER, a vesicle forms in the transitional elements and includes proteins to be exported, but excludes resident proteins of the ER lumen, such as BiP (20). The coat that causes the vesicle to form is now known as COPII. Yeast COPII contains four subunits, sec31p, sec13p, sec23p, and sec24p (see sec mutants). Assembly of a COPII coat requires a small GTPase, Sar1, and a guanine nucleotide exchange factor, Sec12p, in the ER membrane (12) (Fig. 4). The coated vesicle leaving the Golgi carries with it a complement of v-SNARE molecules (see Exocytosis) to allow it to fuse with the cis-Golgi network. In yeast, these are Sec22p, Bos1p, and Bet1p. Resident proteins such as BiP may be excluded from the lumen of the coated vesicle because they are oligomerized into complexes that are too big to enter the small vesicle. To some extent, exported proteins are those that lack a retention signal and so are not retained in the ER. Export of secreted proteins would then be by default, because they lack information to go anywhere else. There is evidence, however, that positive sorting occurs (21) (Fig. 5). In yeast, the secreted protein invertase is recognized by a membrane-bound ER protein (Emp24p) that is required for its transport to the Golgi (22). Furthermore, cargo proteins are concentrated as they leave the ER (23, 24). Since most soluble resident proteins in the ER lumen are not glycosylated, an attractive hypothesis is that exported proteins are recognized by a lectin, which concentrates them in budding vesicles. A protein, ERGIC-53, recycles between the ER and the Golgi and is a lectin with the capacity to bind the mannose residues found on newly synthesized secretory proteins (25). Proteins such as ERGIC-53 might bind secreted proteins and actively carry them to the Golgi complex, in the same way that the mannose phosphate receptor carries newly formed lysosomal enzymes to the prelysosomal compartment.
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Fusion of the ER-derived carrier vesicle with the Golgi membranes requires the v-SNAREs Sec22p, Bos1p, and Bet1p in the vesicle membrane, and the cognate cis-Golgi t-SNARE, the sed5 protein. In addition, a second class of small GTPases are required that are members of the rab family (26). The rab GTPases are ras-like proteins that are specific to a certain organelle or a certain trafficking step. In yeast, mutations in the rab protein ypt1 inhibit fusion with the Golgi and cause the accumulation of carrier vesicles. The ypt1 protein appears to play a role in v-SNARE/t-SNARE interactions (see Exocytosis) involved in cis-Golgi targeting. Two of the v-SNAREs in yeast, Bos1p and Sec22p, normally form a complex. The complex does not form if ypt1 is absent, nor is a v-SNARE/t-SNARE complex formed, suggesting that ypt1 plays a role in activating the v-SNAREs, allowing a v-SNARE/t-SNARE complex to form. It is also possible, however, that ypt1 is also activating the t-SNARE complex on the target membrane, by removing its chaperone, Sly1p. This chaperone is a member of the Sec 1p family of syntaxin-family Chaperones (see Exocytosis).