Function of Cap-Binding Proteins

The function of the cap group on mRNAs and presumably, therefore, of the cap recognition proteins has been probed by the construction of mRNAs with altered structures, as well as by analysis of mRNA function in poliovirus-infected cell extracts which lack capbinding activity. Morgan and Shatkin (1980) and Kozak (1980a,ft) prepared reovirus mRNAs with reduced secondary structure by sub- 

Lee et al. (1983) attempted to directly measure the effect of mRNA secondary structure on its reaction with cap-binding proteins. The 50,000- and 80,000-dalton, ATP/Mg -dependent CBPs in crude rabbit reticulocyte ribosomal salt wash showed the usual ATP-de-pendence for crosslinking to the oxidized cap structure of native reovirus mRNA. However, when inosine-substituted mRNA was used, specific crosslinking of these polypeptides occurred in the absence of ATP/Mg . These results were contradicted in a subsequent report (Tahara et al., 1983) which demonstrated that ATP was required for crosslinking purified eIF-4A and 4B to oxidized, inosine-substituted mRNA as well as for authentic ribosome binding by the denatured RNA. The reason for the discrepant results by these two laboratories has not yet been elucidated. Perhaps small amounts of ATP were 

A possibly related observation is that polio-infected cell extracts which do not translate capped mRNAs under reaction conditions optimal for translation by uninfected extracts will perform translation if the salt concentration is reduced (Hansen and Ehrenfeld, unpublished observations). Several investigators have demonstrated a lower salt optimum for translation of chemically or enzymatically uncapped mRNAs (Brown et al., 1982), and have reported that the importance 

At present, the function of the cap group and of the cap-binding proteins is suggested only from indirect experimental evidence, and is still a matter of much speculation. Fortunately, several well-qualified laboratories are directing major efforts toward this problem, and we can anticipate a much improved understanding in the near future. 

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