Cap-independent translation initiation in Xenopus oocytes

Abstract

Eukaryotic cellular mRNAs contain a cap at their 5'-ends, but some viral and cellular mRNAs bypass the cap-dependent mechanism of translation initiation in favor of internal entry of ribosomes at specific RNA sequences. Cap-dependent initiation requires intact initiation factor elF4G (formerly elF-4γ, elF-4Fγ or p220), whereas internal initiation can proceed with elF4G cleaved by picornaviral 2A or L proteases. Injection of recombinant coxsackievirus B4 protease 2A into Xenopus oocytes led to complete cleavage of endogenous elF4G, but protein synthesis decreased by only 35%. Co-injection of edeine reduced synthesis by > 90%, indicating that elf4G-independent synthesis involved ongoing initiation. The spectrum of endogenous proteins synthesized was very similar in the presence or absence of intact elF4G. Translation of exogenous rabbit globin mRNA, by contrast, was drastically inhibited by elf4G cleavage. The N-terminal cleavage product of elF4G (cp(N)), which binds elF4E, was completely degraded within 6-12 h, while the C-terminal cleavage product (cp(C)), which binds to elF3 and elF4A, was more stable over the same period. Thus, translation initiation of most endogenous mRNAs in Xenopus oocytes requires no elF4G, or perhaps only cp(C), suggesting a cap-independent mechanism.