A common function for mRNA 5' and 3' ends in translation initiation in yeast.

Abstract

The mRNA poly(A) tail and its associated poly(A) binding protein (Pab1p) are ubiquitous in eukaryotes. The function of the poly(A) tail is to stabilize mRNA and to stimulate its translation. The development of a poly(A)- and cap-dependent yeast in vitro translation system has allowed us to understand how poly(A) stimulates translation. We find that Pab1p but not the cap binding protein eIF-4E is required for poly(A) tail-dependent translation, and that the Pab1p-poly(A) tail complex functions to recruit the 40S ribosomal subunit to the mRNA. These data introduce a new step into the pathway of translation initiation and merge the translational functions of the two ends of mRNA.

Starting the protein synthesis machine: eukaryotic translation initiation

“…(69)(70)(71)(72) In a yeast cell-free system, the poly(A) tail, like the cap structure, was able to support the recruitment of the 40S ribosomal subunit by itself to an uncapped mRNA. (73) This function of the poly(A) tail as well as the functional synergy with the cap structure requires the poly(A)-binding protein (PABP) (73) and its interaction with the N-terminal part of eIF4G (74,75) (Fig. 3).…”

Conducting the initiation of protein synthesis: the role of eIF4G

“…This stimulation requires PABP and correlates with increased 40 S ribosomes recruitment to the 5' end of the mRNA (Tarun and Sachs, 1995). In yeast, this 5'-3' interaction is not necessary for cell viability as mutations within the PABP binding site of eIF4G only became lethal when the eIF4E binding domain was also disrupted (Tarun et al, 1997).…”

Deciphering the cellular pathway for transport of poly(A)-binding protein II

“…Comparison of mammalian and yeast pre-mRNA 39-end-processing components reveals that, although many homologies can be found, the proteins are not function-ally interchangeable (for a recent review, see )+ An apparent case of divergence during evolution occurs with poly(A)-binding proteins+ Yeast contains a single poly(A)-binding protein (termed Pab1p), which is an essential protein present in both the nucleus and the cytoplasm (Sachs et al+, 1986)+ In the nucleus it stimulates polyadenylation and controls poly(A) tail length (Amrani et al+, 1997;MinvielleSebastia et al+, 1997;Brown & Sachs, 1998), whereas in the cytoplasm it is involved in translation initiation (Sachs & Davis, 1989;Sachs & Deardorff, 1992;Tarun & Sachs, 1995, 1996Le et al+, 1997) and mRNA degradation (Caponigro & Parker, 1995;Boeck et al+, 1998;Coller et al+, 1998)+ In contrast, mammalian cells contain two distinct proteins called poly(A)-binding protein I (PABP1) and poly(A)-binding protein II (PAB II or PABP2)+ PABP1 is predominantly detected in the cytoplasm (Görlach et al+, 1994), whereas PABP2 is localized in the nucleus (Wahle, 1991;Krause et al+, 1994)+ PABP1, which is the mammalian counterpart of yeast Pab1p (51% identity), is apparently involved in both cytoplasmic mRNA stability (Bernstein et al+, 1989;Wormington et al+, 1996;Afonina et al+, 1997;Ford et al+, 1997;Körner & Wahle, 1997) and translation (Craig et al+, 1998), but to date there is no evidence that it participates in nuclear pre-mRNA 39-end processing (see )+ Consistent with its nuclear localization, PABP2 is the mammalian protein involved in polyadenylation+ PABP2 stimulates processive poly(A) addition and controls the size of the tail to be ;250 nt in length (Wahle, 1991(Wahle, , 1995Bienroth et al+, 1993)+ How PABP2 bound to poly(A) tails in the nucleus is replaced by PABP1 in the cytoplasm remains unknown, but recent evidence indicates that both PABP1 and PABP2 shuttle between nucleus and cytoplasm (Afonina et al+, 1998;Chen et al+, 1999)+ Because it is not known when PABP1 exchanges with PABP2, one possibility is that PABP2 crosses the nuclear pores in association with the mRNA and dissoci...…”