378 HiciiAiu) sciiwKKr and .lonx hisikh' 



(^tlior inocli;inisins can he pi'opo.-cd hy wliidi the iiicoiiiiiio; amino 

 acid could recognize that the |)rece(nn^ amino acid is j)resent. Whatever 

 mechanism is iiostulated, however, it appeal's likely that the important 

 characteristic of chain growth fi'om the N-terminal end is that the 

 incoming amino acid and the i)i'('ccding one are both on the template. 

 This provides steric conditions suitable for jieptide bond formation, and 

 could also ensure uninterrupted chain growth, as discussed above. 



3. Final Stages in Protein Sj/nthesis 



The pre^•ious discussion has dealt with indirect e\-idence for a particu- 

 lar sequential type of peptide chain growth. It should be noted that 

 details of the mechanism of peptide bond formation and the inter- 

 mediates directly involved in this process are as yet unknown. The role 

 for GTP suggested eai'liei-, which is hypothetical as yet, is the first 

 approach to this problem. Skipping over these basic questions, therefore, 

 the next problem concerns the mechanism by which finished proteins 

 are released from the ribosome. The best evidence that some mechanism 

 is needed to release finished proteins from their site of synthesis comes 

 from experiments where "nascent," enzymatically active proteins on 

 ribosomes cannot be removed by simple washing techniques (Kihara 

 et a!.. 1961; Cowie et al, 1961; Siekevitz and Palade, 1960). 



In cell-free systems, Simkin (1958) showed that labeled liver 

 microsomes formed soluble protein when incubated with cell sap and an 

 energy-generating system. In the reticulocyte ribosome system, similar 

 studies demonstrated the formation of labeled, soluble hemoglobin when 

 labeled ribosomes were incubated in the complete system (C^--amino 

 acids, energy, and enzymes). It was also shown that the ribosomes were 

 not degraded under these conditions (]\Iorris and Schweet, 1961). How- 

 ever, these experiments are essentially "chase" experiments and do not 

 distinguish between release of completed proteins and peptide bond 

 synthesis which results in synthesis of completed chains. Soluble enzyme 

 fractions which increase the formation of soluble protein without increas- 

 ing total incorporation greatly have been termed "releasing" enzymes 

 (Lamfrom, 1961), and it has been suggested that these were species 

 specific. Howc\-(M-, if i-ibosomal-bound proteins were nearly completed, 

 the addition of only a few amino acids might result in tlie fonnation of 

 soluble protein. Further discussion and evidence for this viewpoint is 

 given in the next section. Until systems are developed where incorpora- 

 tion into ribosomes is 7wt occurring while "release" is being measured, 

 the problem of whether or not specific "releasing" enzymes exist remains 

 open. 



When the transfer system (from amino acyl-RNA to i)rotein) is 



