Proteins poly A binding

Schonfelder, M., Horsch, A. Schmid, H.-P. (1985). Heat shock increases the synthesis of the poly(A)-binding protein in HeLa cells. Proceedings of the National Academy of Sciences, USA, 82, 6884-8. 

Kahvejian, A., Svitkin, Y. V., Sukarieh, R., M Boutchou, M. N., and Sonenberg, N. (2005). Mammalian poly(A)-binding protein is a eukaryotic translation initiation factor, which acts via multiple mechanisms. Genes Dev. 19, 104—113. 

Imataka, H., Gradi, A., and Sonenberg, N. (1998). A newly identified N-terminal amino acid sequence of human eIF4G binds poly(A)-binding protein and functions in poly(A)-dependent translation. EMBOJ. 17, 7480-7489. 

Kuhn, U., and Wahle, E. (2004). Structure and function of poly(A) binding proteins. 

Scheuermann, T., Schulz, B., Blume, A., Wahle, E., Rudolph, R., and Schwarz, E. (2003). Trinucleotide expansions leading to an extended poly-L-alanine segment in the poly (A) binding protein PABPN1 cause fibril formation. Protein Sci. 12, 2685-2692. 

Gray NK, Coller JM, Dickson KS, Wickens M (2000) Multiple portions of poly(A)-binding protein stimulate translation in vivo. EMBO J 19 4723-4733... 

Uchida N, Hoshino S, Imataka H, Sonenberg N, Katada T (2002) A novel role of the mammalian GSPT/eRE3 associating with poly(A)-binding protein in Cap/Poly(A)-dependent translation. J Biol... 

In vitro experiments show that correct modification of the 3 -end requires at least three protein factors the CPSF protein, the poly-A polymerase and the poly-A binding protein. The CPSF protein (CPSF cleavage and polyadenylation specificity factor) binds to the AAUAA signal and brings the poly-A polymerase to the polyadenylation site. The poly-A polymerase is supported by the poly-A binding protein. The latter binds to the poly-A sequence and is required for the transition from the phase of synthesis of short poly-A sequences to the formation of mature poly-A sequences (ca. 200 A-residues). 

FIGURE 27-22 Protein complexes in the formation of a eukaryotic initiation complex. The 3 and 5 ends of eukaryotic mRNAs are linked by a complex of proteins that includes several initiation factors and the poly(A) binding protein (PAB). The factors elF4E and elF4G are part of a larger complex called elF4F. This complex binds to the 40S ribosomal subunit. 

Most pre-mRNA transcripts are cleaved post-transcriptionally near the 3 end between a polyadenylation signal (5 -AAUAAA-3 ) and 5 -YA-3 (where Y = a pyrimidine). A GU-rich sequence may also be located further downstream. Specific proteins bind to these sequence elements to form a complex. One of the bound proteins, poly(A) polymerase, then adds a poly(A) tail of up to 250 A residues to the new 3 end of the RNA molecule and poly(A) binding protein molecules bind to this. The poly(A) tail protects the 3 end of the final mRNA against nuclease degradation and also increases translational efficiency of the mRNA. Some pre-mRNAs (e.g. histone pre-mRNAs) are cleaved near the 3 end but no poly(A) tail is added. 

Kerekatte, V., Keiper, B.D., Badorff, C., Cai, A., Knowlton, K.U., and Rhoads, R.E. (1999). Cleavage of poly(A)-binding protein by coxsackievirus 2A protease in vitro and in vivo another mechanism for host protein synthesis shutoff J Virol 73, 709-717. 

Sequences of poly(A) binding protein H, PABP2 (AF026029)... 

In the presence of purified poly(A)-binding protein I (PABPI), elF4E, and elF4G, eukaryotic mRNAs form circular structures, visible in this force-field electron micrograph. In these structures, protein-protein and protein-mRNA interactions form a bridge between the 5 and 3 ends of the mRNA as diagrammed in Figure 4-31. [Courtesy of A. Sachs.]... 

How would a mutation in the poly (A)-binding protein I gene affect translation How would an electron micrograph of polyribosomes from such a mutant differ from the normal pattern ... 

A multiprotein complex that includes poly(A) polymerase (PAP) carries out the cleavage and polyadenylatlon of a pre-mRNA. A nuclear poly (A)-binding protein, PABPII, stimulates addition of A residues by PAP and stops addition once the poly(A) tail reaches 200-250 residues (see Figure 12-4). 

Another participant in mRNP transport to the cytoplasm Is the nuclear cap-bIndIng complex, mentioned earlier as protection against exonuclease attack on the 5 end of nascent transcripts and pre-mRNAs. Electron microscopy experiments discussed below have demonstrated that the 5 end of mRNAs lead the way through the nuclear pore complex. Recent experiments in yeast indicate that the 3 poly(A) tail plays an Important role In mRNP transport, suggesting that a poly(A)-binding protein participates. Nucleoporins associated with the NPC cytoplasmic filaments In addition to FG-nucleoporins are required for mRNA export and may function to dissociate the mRNA-exporter and other mRNP proteins that accompany the mRNP through the pore. 

Once the mRNP reaches the cytoplasm, most of the mRNP proteins that associated with the mRNA in the nucleus, the nuclear cap-binding complex, and the nuclear poly (A)-binding protein (PABPII) dissociate and are shuttled back to the nucleus. In the cytoplasm, the 5 cap of an exported mRNA is bound by the eIF4E translation initiation factor, the poly(A) tail is bound by multiple copies of the cytoplasmic poly(A)-binding protein (PABPI), and other RNA-binding proteins associate with the body of the mRNA, forming a cytoplasmic mRNP that has a lower ratio of protein to RNA than nuclear mRNPs. 

This mature mRNA complexes with the poly(A) binding protein and other proteins. It travels through pores in the nuclear envelope into the cytoplasm. There it combines with ribosomes and directs the incorporation of amino acids into proteins. 

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