12.4Protein Transport in Prokaryotic Cells

Fig. 12-13 Intracellular protein transport as observed in prokaryotic cells

A ribosome is led to the cell membrane by SRP, where it is known to either synthesize a protein, secrete it, and then incorporate it into a membrane (left) or occassionally to bind it to a translocon via a signal sequence receptor, secrete it, and then incorporate it into the membrane (right).

In prokaryotic cells lacking organelles such as ER or Golgi body, secretion of proteins outside the cell and uptake of membrane proteins into the cell membrane are performed through translocons distributed throughout the cell membrane. These make use of two different methods (Fig. 12-13). One is a method where the ribosome binds to the cell membrane to synthesize secretory and membrane proteins (as observed in ER of eukaryotic cells). Prokaryotic cells also have SRPs, which play a role in guiding the ribosome to a translocon in the cell membrane. The other method is one in which the signal sequence found in a protein is recognized by a receptor in the cell membrane, and the protein is then transported outside the cell or absorbed into the cell. This is analogous to the eukaryotic method of transporting proteins synthesized by free polysomes.

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