Figure 226

Anterograde and retrograde transport between the rER and c/s-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 complex, respectively. COP-II is involved in anterograde transport from the rER to the c/s-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.

• COP-II is responsible for anterograde transport, forming rER transport vesicles destined for the CGN (Fig. 2.27b). 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

Cytoplasmic basophilia is also 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.

Basophilia in these cells was formerly called ergasto-plasm and is due to the presence of large amounts of RNA.

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