118 KERATIN AND KERATINIZATION 



presence of ATP the radioactivity was partially transferred to the haemo- 

 globin fraction. Hendler (1957), and Simkin and Work (1958) have 

 reported similar findings. Fig. 51, adapted from Stephenson et al. (1959), 

 summarizes the several stages of protein synthesis as outlined above. 



1. Activation of Amino Acids 



AA + ATP + E ^± [AA ~ AMP] - E + PP 



2. Addition of Nucleotide End Group to S-RNA 



- RNA + ATP + CTP «± [RNA - pCpCpA] + PP 



3. Binding of Amino Acid to this RNA 



AA + ATP + [RNA - pCpCpA] ^ [RNA - pCpCpA - AA] + PP 



4. Transfer of Amino Acids to RNP Particles, Binding to RNA of 

 Particles Followed by Polymerization of Amino Acids 



[RNA - pCpCpA - AA] + ATP + GTP + RNP 



(Particles) + Soluble fraction (?) 

 — > [polypeptide chain — particle] 



5. Release of Bound Polypeptide and Completion of Protein by Cross- 

 linking and Secondary Bonding 



AA amino acids 



E enzyme (many specific enzymes known) 

 PP pyrophosphate 

 CTP Cytosine triphosphate 

 S-RNA " soluble " RNA 

 pCpCpA nucleotide end group 



] units linked as complex 



Fig. 51. A current scheme showing possible steps in protein synthesis. 



A special problem, arising in cells which are secreting their formed 

 protein, is the relation of the microsomal RNA particles to the system of 

 membranes found in these cells (see Fig. 49). Probably the microsomal 

 protein is released to the membranes and accumulates within them (Palade 

 and Siekevitz, 1956) as can often be noted in micrographs. In keratin- 

 forming cells and other retaining cells, membranes are not involved 

 and the newly-formed protein appears to be released directly into the cell 

 sap (Plates 9 and 11). 



