The protein synthetic system of the cell consists of the rough endoplasmic reticulum and ribosomes.
The cytoplasm of a variety of cells engaged chieﬂy in protein synthesis stains intensely with basic dyes. The basophilic staining is caused by the presence of RNA. That portion of the cytoplasm that stains with the basic dye is called ergastoplasm. The ergastoplasm in secretory cells (e.g., pancreatic acinar cells) is the light microscopic image of the organelle called the rough endoplasmic reticulum (rER).
With the TEM, the rER appears as a series of interconnected, membrane-limited, ﬂattened sacs called cisternae, with particles studding the exterior surface of the membrane (Fig. 2.25). These particles, called ribosomes , are attached to the membrane of the rER by ribosomal docking proteins. Ribosomes measure 15 to 20 nm in diameter and consist of a small and large subunit. Each subunit contains ribosomal RNA (rRNA) of different length as well as numerous different proteins. In many instances, the rER is continuous with the outer membrane of the nuclear envelope (see the next section). Groups of ribosomes form short spiral arrays called polyribosomes or polysomes (Fig. 2.26) in which many ribosomes are attached to a thread of messenger RNA (mRNA).
Protein synthesis involves transcription and translation.
The production of proteins by the cell begins within the nucleus with transcription, in which the genetic code for a protein is transcribed from DNA to pre-mRNA. After posttranscriptional modifications of the pre-mRNA molecule— which includes RNA cleavage, excision of introns, rejoining of exons, and capping by addition of poly(A) tracks at the
FIGURE 2.24 • Proteasome-mediated degradation. This degradation pathway involves tagging proteins destined for destruction by a polyubiquitin chain and its subsequent degradation in proteasome complex with the release of free reusable ubiquitin molecules. Ubiquitin in the presence of ATP is activated by a complex of three ubiquitin-activating enzymes (E1, E2, and E3) to form a single polyubiquitin chain that serves as the degradation signal for the 26S proteasome complex. The regulatory particle (19S RP) that forms the lid of the main protein destruction chamber (20S core particle) recognizes polyubiquitin tags, unfolds the protein, and inserts and regulates its entry into the destruction chamber. The regulatory particle on the opposite side of the chamber releases short peptides and amino acids after degradation of the protein is completed. Free ubiquitin molecules are released by de-ubiquitinating enzymes (DUBs) and recycled.
3 end, and a methylguanosine cap [M(7) GPPP] at the 5 end—the resulting mRNA molecule leaves the nucleus and migrates into the cytoplasm (Fig. 2.27). Transcription is followed by translation in which the coded message contained in the mRNA is read by ribosomal complexes to form a polypeptide. A typical single cytoplasmic mRNA molecule binds to many ribosomes spaced as close as 80 nucleotides apart, thus forming a polyribosome complex, or
FIGURE 2.25 • Electron micrograph of the rER. This image of the rER in a chief cell of the stomach shows the membranous cisternae (C) closely packed in parallel arrays. Polyribosomes are present on the cytoplasmic surface of the membrane surrounding the cisternae. The image of a ribosome-studded membrane is the origin of the term rough endoplasmic reticulum. A few ribosomes are free in the cytoplasm. M, mitochondrion. 50,000.