Differential inhibition with partially purified and endogenous rabbit reticulocyte globin mRNA by 7-methylguanosine 5'-monophosphate

Proteins associated with rabbit reticulocyte mRNA caps during translation as investigated by photocrosslinking

This laboratory previously detected by UV crosslinking a number of proteins associated with cytoplasmic mRNA in mammalian cells, and the data suggested that they are involved in translation. To find out which proteins are associated with caps we made use of reticulocyte mRNA specifically labeled in the cap with 32P together with a cell-free translation system and UV crosslinking. Approximately 8 bands corresponding to proteins crosslinked to the cap itself have been detected by polyacrylamide gel electrophoresis after UV crosslinking and digestion with RNases or tobacco pyrophosphatase. All but one were specific for methylated caps. One was similar in size and partial peptide map to a cap-binding protein, CBP I, previously identified in other laboratories, and most of the others corresponded to proteins previously known to be associated with mRNA but not known to be associated with caps. The results suggest that most mRNA-associated proteins are associated with caps or poly(A). Also, the number of cap-associated proteins may be greater than previously suspected.

Effects of cap analogue or cap removal on the translation of rat brain mRNA in vitro

The role of cap structures in the translation of brain mRNA was examined by measuring protein biosynthesis in vitro in wheat germ and reticulocyte systems programmed by mRNA that was either untreated or oxidized by periodate or from which 5'-terminal 7-methylguanosine (m7G) was removed by oxidation and beta-elimination. In another series of reactions, amino acid incorporation into polypeptides was measured in the absence and in the presence of varying concentrations of the cap analogue 7-methylguanosine 5'-triphosphate (pppm7G). The results indicated that any of the above treatments interfered with brain mRNA translation, the degree of inhibition depending on the translation system used, the concentration of mRNA, and the source of initiation factors. Homologous brain initiation factors were superior to reticulocyte factors in providing a partial relief from inhibition of translation caused by these treatments. It was also found that synthesis of the brain-specific protein S-100 was inhibited by beta-elimination of mRNA, by pppm7G, or by the presence of capped globin mRNA, indicating that the mRNA for this protein was probably capped.

The effect of hemin, adenosine 3',5'-monophosphate and phosphorylated sugars on Met-tRNAfMet deacylase activity in rabbit reticulocyte lysates

The effect of hemin, phosphorylated sugars, adenosine 3',5'-monophosphate (cyclic AMP) and a number of purines on a specific initiator tRNA deacylase activity in rabbit reticulocytes has been investigated. In the concentration range established to be optimal for maximal stimulation of translation (5.5-30.0 microM), hemin produces a 20-82% inhibition of Met-tRNAfMet deacylation. In contrast, all phosphorylated sugars tested, with the exception of fructose 1,6-diphosphate, are without effect. High concentrations of cyclic AMP (2-4 mM) also significantly inhibit the deacylase activity. The role of hemin and Met-tRNAfMet deacylase in the control of peptide initiation are discussed.

Inhibitory effects of 'cap' analogues on globin mRNA and encephalomyocarditis RNA translation in a reticulocyte cell-free system

The cap analogues 7-methylguanosine 5'-phosphate [m7G(5')p], 7-methylguanosine 5'-triphosphate [m7G(5')ppp] and 2'-O-methylguanosine 5'-triphosphate [Gm(5')ppp] inhibit the translation of capped globin mRNA and encephalomyocarditis (EMC) RNA (a naturally uncapped mRNA) in a reticulocyte cell-free system. This inhibition occurs at the level of protein synthesis initiation and is of a competitive type since it can be overcome by increasing the mRNA concentration. However, the translation of globin mRNA is more sensitive to the inhibitory effects of the cap analogues m7G(5')p and m7G(5')ppp than translation of EMC RNA. The same spectra of specific inhibition is also observed with some other initiation inhibitors such as aurintricarboxylic acid, which inhibits mRNA binding, but not with pactamycin which does not affect mRNA interaction. A model is presented suggesting that this preferential inhibition by cap analogues could be explained mainly by the different affinities of globin mRNA and EMC RNA for the initiation complexes between 40-S subunits and Met-tRNAf. Moreover Gm(5')ppp cannot be considered simply as a cap analogue since it also affects some step prior to mRNA binding.

5'-Terminal 7-methylguanosine and mRNA function. The effect of enzymatic decapping and of cap analogs on translation of tobacco-mosaic-virus RNA and globin mRNA in vitro

1. Decapped tobacco mosaic virus (TMV) RNA and rabbit globin mRNA were prepared by enzymic treatment of RNAs with nucleotide pyrophosphatase purified from potato. The extent of removal of 5'-terminal 7-methylguanosine 5'-monophosphate (m7GMP) from TMV RNA was at least 97% as estimated by labeling of the 5' termini in vitro with S-adenosyl[methyl-3H]methionine catalysed by vaccinia virus methyltransferases. 2. The effect of enzymic decapping was compared with the effect of cap analogs on mRNAs translation in a nuclease-treated rabbit reticulocyte lysate and in a wheat germ extract. When translation was studied at low K+ concentration, little or no dependence on 5'-terminal 7-methylguanosine was found with either cell-free system. The importance of the 5'-terminal cap for the efficient translation of TMV RNA and globin mRNA increased as the concentration of K+ in a protein-synthesis system was raised. In a reticulocyte lysate analogs and enzymic decapping had a similar effect on translation. In a wheat germ extract, mRNA decapping resulted in a more pronounced decrease of mRNA activity, presumably due to the increased susceptibility of decapped mRNAs to the nucleases present in this protein synthesis system. 3. The requirement for a 5'-terminal cap was similar for the synthesis of 130,000-Mr and 165,000-Mr polypeptides coded by TMV RNA. This indicates that both proteins may be initiated at the common site close to the 5' terminus.

Inhibition of translation of lens mRNAs in a messenger dependent reticulocyte lysate by cap analogues

The nuclease treated reticulocyte lysate forms a highly efficient and completely mRNA-dependent cell-free system. In this system the functioning of the cap on eukaryotic mRNAs was explored by blocking cap recognition with cap analogues. Translation of capped mRNAs was severely inhibited, while translation of uncapped mRNAs was unaffected. It is concluded that this cell-free system can be used for screening cap dependence in the translation of specific mRNAs, like calf lens mRNAs. At 1.2 mM m7G5'p, 0.16 mM m7G5'pp or 0.16 m7G5'ppp5'G, translation of all lens mRNAs was totally inhibited. At lower concentrations the sensitivity to cap analogues was different for the various species of lens crystallin messenger. gamma-Crystallin mRNA showed relatively the lowest response. The translation of added polyribosomes was also inhibited by the cap analogue. It is concluded that translation of all crystallin messengers is cap-dependent.

The effect of the messenger RNA concentration on the competitive inhibition of translation by cap-analogues

Inhibition of translation of several mRNA species in a micrococcal nuclease treated reticulocyte lysate by cap analogues was compared with the competition between two mRNAs. Inhibition characteristics were very similar, only complete mRNA molecules inhibited at concentrations 150 times lower than m7 G5'ppp5'G. The inhibition of mRNA translation by cap analogues could be neutralized by the addition of extra mRNA in a manner predicted from the competitive nature of the inhibition by cap analogues.

Diminished sensitivity of re-initiation of translation to inhibition by cap analogues in reticulocyte lysates

In a nuclease-treated reticuloyte lysate reinitiation of protein biosynthesis was less inhibited by 7-methylguanosine 5'-monophosphate (m7GMP), whereas primary-initiation could be totally blocked by cap analogues. We suggest that a cap binding factor binds to mRNA as a first step in initiation of protein synthesis and that this factor remains bound during the subsequent reinitiation cycles. Primarily because of the greater relative importance of the reinitiation process, globin synthesis in the untreated lysate was less inhibited by m7GMP.