Proteins that are to be used within the cell are likely to be produced by polyribosomes that float freely in the cytoplasm, unattached to other organelles. If the protein is to be secreted by the cell, however, it is made by mRNA-ribosome complexes that are located on the granular endoplasmic reticulum. The membranes of this system enclose fluid-filled spaces called cis-ternae, into which the newly formed proteins may enter. Once in the cisternae, the structure of these proteins is modified in specific ways.
When proteins destined for secretion are produced, the first thirty or so amino acids are primarily hydrophobic. This leader sequence is attracted to the lipid component of the membranes of the endoplasmic reticulum. As the polypeptide chain elongates, it is "injected" into the cisterna within the endoplasmic reticulum. The leader sequence is, in a sense, an "address" that directs secretory proteins into the endoplasmic reticulum. Once the proteins are in the cisterna, the leader sequence is en-zymatically removed so that the protein cannot reenter the cytoplasm (fig. 3.24).
The processing of the hormone insulin can serve as an example of the changes that occur within the endoplasmic reticu-lum. The original molecule enters the cisterna as a single polypeptide composed of 109 amino acids. This molecule is called preproinsulin. The first twenty-three amino acids serve as a leader sequence that allows the molecule to be injected into the cisterna within the endoplasmic reticulum. The leader sequence is then quickly removed, producing a molecule called proinsulin. The remaining chain folds within the cisterna so that the first and last amino acids in the polypeptide are brought close together. Enzymatic removal of the central region produces two chains—one of them, twenty-one amino acids long; the other, thirty amino acids long—that are subsequently joined together by disulfide bonds (fig. 3.25). This is the form of insulin that is normally secreted from the cell.
■ Figure 3.23 The translation of messenger RNA (mRNA). As the anticodon of each new aminoacyl-tRNA bonds with a codon on the mRNA, new amino acids are joined to the growing tip of the polypeptide chain.
■ Figure 3.24 How secretory proteins enter the endoplasmic reticulum. A protein destined for secretion begins with a leader sequence that enables it to be inserted into the cisterna (cavity) of the endoplasmic reticulum. Once it has been inserted, the leader sequence is removed and carbohydrate is added to the protein.
Cell Structure and Genetic Control 69
■ Figure 3.25 The conversion of proinsulin into insulin. The long polypeptide chain called proinsulin is converted into the active hormone insulin by enzymatic removal of a length of amino acids (shown in gray). The insulin molecule produced in this way consists of two polypeptide chains (red circles) joined by disulfide bonds.
Secretory proteins do not remain trapped within the granular endoplasmic reticulum. Instead, they are transported to another organelle within the cell—the Golgi complex (or Golgi apparatus), as previously described. This organelle serves three interrelated functions:
1. Proteins are further modified (including the addition of carbohydrates to form glycoproteins) in the Golgi complex.
2. Different types of proteins are separated according to their function and destination in the Golgi complex.
3. The final products are packaged and shipped in vesicles from the Golgi complex to their destinations (see fig. 3.13).
In the Golgi complex, for example, proteins that are to be secreted are separated from those that will be incorporated into the cell membrane and from those that will be introduced into lysosomes. Each is packaged in different membrane-enclosed vesicles and sent to its proper destination.
Test Yourself Before You Continue
1. Explain how mRNA, rRNA, and tRNA function during the process of protein synthesis.
2. Describe the granular endoplasmic reticulum and explain how the processing of secretory proteins differs from the processing of proteins that remain within the cell.
3. Describe the functions of the Golgi complex.
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