mRNA encoding a cytosolic protein free polyribosome Figure 14-12 Free and membrane-bound remains free in cytosol in cytosol ribosomes. A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are transported into membrane-bounded 2 organelles, including the ER. It is the ER signal sequence on a growing polypeptide chain that directs the ribosome making common pool of ribosomal subunits in cytoso the protein to the ER membrane. The mRNA molecule may remain bound to the 3 ER ER as part of a polyribosome, while the signal ribosomes that move along it are recycled quence at the end of each round of protein syn mRNA encoding a protein polyribosome bound to ER targeted to ER remains thesis, the ribosomal subunits are released membrane-bound membrane by multiple growing polypeptide chains and rejoin the common pool in the cytosol ER membrane20