10.2.2—
Free and Membrane-Bound Polysomes

Ribosomes associate with mRNA to form polysomes, as can be seen in the electron micrograph (Fig. 10.1); the size of the polysome varies according to the length of the mRNA and the number of attached ribosomes. Polysomes are found either free in the cytoplasm or attached to the surface of membranes of the endoplasmic reticulum (ER) and the nucleus. In animal embryonic cells, most of the polysomes are free and engaged in protein synthesis for internal use, whereas in those differentiated animal cells from which large amounts of protein are exported, most of the polysomes are found attached to the ER. Thus, the generalization arose that membrane-bound ribosomes synthesize protein for export and free ribosomes for intracellular use. It soon became clear however, that membrane-bound ribosomes occur in some tissues which do not export protein, and that in cells where all the protein synthesized is for internal use, different classes of protein are synthesized on the two types of ribosomes. Less information is available for plants. In the developing broadbean seed the highly vacuolate, relatively membrane-free cells of the cotyledon are transformed just prior to the onset of storage protein synthesis, into cells whose cytoplasm is vesicular and in which ER with attached ribosomes is very prominent. This new protein synthesis machinery is assembled at a precise time in the course of seed development for the production, in large amounts, of the storage proteins of the seed (Bailey et al., 1970; see Fig. 10.2). Later, during dehydration of the seed, ribosomes become detached and the population of free ribosomes thereby increased. A similar series of events has been recorded for the developing seeds in many other plants.

The way in which ribosomes attach to membranes is not clear; the ribosomes themselves, the membranes and the protein being synthesized, have all been suggested as being involved in the binding. There is evidence from animals that the large subunit is in close contact with the membranes, and Baglioni et al. (1971) have postulated that the large subunit binds directly to the membrane, probably at a specific binding site (Sunshine et al., 1971), and that a protein on

Figure 10.1
Electron micrograph of a thin section of parts of two adjacent cells in a
shoot apex of pea showing ribosomes and polyribosomes; CW = Cell Wall, Pl
= Plasmalemma, N = Nucleus, Pm = Membrane bound polysomes, R = Ribosome
Ph = Polysome helix. By courtesy of A. D. Greenwood, Department of Botany
and Plant Technology, Imperial College of Science and Technology, London.

the membranes is responsible for the attachment (James et al., 1969). Alternatively, it has been suggested that one of the proteins of the large subunit is responsible, whereas other observations suggest that the binding may be dependent on the nascent polypeptide chain. It now seems likelly that, in vivo, membrane-bound ribosomes synthesize a different class of protein to free ribosomes, whereas in vitro, proteins of both classes are synthesized by both types of ribosomes, suggesting the control is not a function of the ribosome itself.