FIGURE 2.28 • Anterograde and retrograde transport between the rER and cis-Golgi network. Two classes of coated vesicles are involved in protein transport to and from the rER. These vesicles are surrounded by COP-I and COP-II protein coat complexes, respectively. COP-II is involved in anterograde transport from the rER to the cis-Golgi network (CGN), and COP-I is involved in retrograde transport from the CGN back to the rER. After a vesicle is formed, the coat components dissociate from the vesicle and are recycled to their site of origin. The COP-I protein coat is also involved in retrograde transport between cisternae within the Golgi apparatus (see Fig. 2.13).
 COP-II is responsible for anterograde transport, forming rER transport vesicles destined for the CGN (Fig. 2.29b). COP-II assists in the physical deformation of rER membranes into sharply curved buds and further separation of vesicles from the rER membrane. Most proteins produced in the rER use COP-II–coated vesicles to reach the CGN.
Shortly after formation of COP-I– or COP-II–coated vesicles, the coats dissociate from the newly formed vesicles, allowing the vesicle to fuse with its target. The coat components then recycle to their site of origin.
“Free” ribosomes synthesize proteins that will remain in the cell as cytoplasmic structural or functional elements.
Proteins targeted to the nucleus, mitochondria, or peroxisomes are synthesized on free ribosomes and then released into the cytosol. In the absence of a signal sequence, proteins that are synthesized on free ribosomes remain in the cytosol. Cytoplasmic basophilia is associated with cells that produce large amounts of protein that will remain in the cell. Such cells and their products include developing red blood cells (hemoglobin), developing muscle cells (the contractile proteins actin and myosin), nerve cells (neurofilaments), and keratinocytes of the skin (keratin). In addition, most enzymes of the mitochondrion are synthesized by free polysomes and transferred into that organelle.
FIGURE 2.29 • Electron micrograph of COP-I– and COP-II–coated vesicles. a. This image shows COP-I–coated vesicles that initiate retrograde transport from the cis-Golgi network to the rER. In this image, taken with a quick-freeze deep-edge microscope, note the structure of the CGN and emerging vesicles. 27,000. b. Image of COP-II–coated vesicles that are responsible for anterograde transport. Note that the surface coat of these vesicles is different from that of clathrin-coated vesicles.
50,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)
Basophilia in these cells was formerly called ergastoplasm and is caused by the presence of large amounts of RNA. In this case, the ribosomes and polysomes are free in the cytoplasm (i.e., they are not attached to membranes of the endoplasmic reticulum). The large basophilic bodies of nerve cells, which are called Nissl bodies, consist of both rER and large numbers of free ribosomes (Fig. 2.30). All ribosomes contain RNA; it is the phosphate groups of the RNA of the ribosomes, not the membranous component of the endoplasmic reticulum, that accounts for basophilic staining of the cytoplasm.
FIGURE 2.30 • Electron micrograph of a nerve cell body showing the rER. This image shows rER profles as well as numerous free ribosomes located between the membranes of the rER. Collectively, the free ribosomes and membrane-attached ribosomes are responsible for the characteristic cytoplasmic basophilia (Nissl bodies) observed in the light microscope in the perinuclear cytoplasm of neurons. 45,000.