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Initiation Factors

Initiation Factors - The initiation of protein synthesis requires certain initiation factors (IF). Initiation factors were discovered when it was found that E. coli ribosomes washed with 0.5M NH4CL could translate synthetic messengers like poly(U) but not natural mRNAs like coliphage RNAs, unless treated with the wash. Obviously the wash contained certain factors which were essential for translation.

Similarly Miller and Schweet (1968) observed that ribosomes of reticulocytes washed with O.5M KCl could not initiate synthesis of globin at low Mg++ concentrations unless the wash was added back. (Washed ribosomes could translate poly(U) at high Mg++ concentrations). In 1970 Anderson and his co-workers isolated two protein factors MI and M2, which restored the translation capacity of washed rabbit reticulocyte ribosomes at low MgH concentrations.

Another factor,M3, was required for the translation of natural messengers like globin mRNA. M2 was later found to consist of two distinct proteins M2A and M2B. IF-M2B further consists of IF-M2Bα and IF-M2β components. In 1972 Levin and his co-workers discovered a wash factor, later termed IF-MP, which could form a ternary complex with met-tRNA and GTP in the absence of the ribosomes

In prokaryotes three factors, IF-1, IF-2 and IF-3 are required for initiation of protein synthesis. These factors are found in the 30S subunit of the ribosome. The molecular weights of these factors are 9,200, 80,000 and 30,000 daltons, respectively. IF-l and IF-2 are required for the binding of initiation tRNA (tRNAmet or tRNAi) to the 30S ribosomal subunit.

IF-2 is involved in the binding of guanosine triphosphate (GTP) and contains the SH groups necessary for this binding. IF-3, a lysine and arginine rich basic protein, is required for the binding of the 30S ribosomal subunit to the initiation sequence of mRNA.

In eukaryotes there is no equivalent of the prokaryotic IF-1. According to the Anderson (1975) nomenclature the following initiation factors are found in eukaryotes: IF-MP, 1F-Ml, IF-M2A, IF-M2Bα,IF-M2B β and IF-M3. The corresponding terminology used by Ochoa (1976) is eIF-2, eIF-2', elF-2a1 eIF-2a2 and elF-2a3.

eIF-2 forms a ternary complex (Met-tRNAi.eIF-2.GTP) with Met tRNAi and GTP. This complex interacts with the 408 ribosomal subunit in the absence of a template to form the 408 initiation complex eIF-2' (IF-M1) promotes the ADG dependent binding of Met tRNAi to the 40S subunit in the absence of GTP.

The accessory factors eIF-2a1, eIF-2a2 and elF-2a3 (IF-M2) are required for the binding of Met-tRNAi along with GTP. In Artemia salina eIF-2A1,2,3 are required fur the formation of the 80S initiation complex after eIF-2 has formed the 40S initiation complex, eIF-3 is a multi protein complex consisting of several polypeptide chains.

For Krebs ascites cells the molecular weights of the light and heavy fractions of eiF-3 are ~ 480,000 and ~ 780,000 respectively. Prokaryotic IF-3 on the other hand consists of a single polypeptide chain. The molecular weights of the α and β forms are 23,000 and 21,000, respectively.

 
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