5 -Terminal cap structure

No sequence studies of the interaction of these proteins with mRNA have yet been reported, and specific binding to a region in mRNA remains to be demonstrated. On the basis of the available data, it is tempting to suggest that eIF-2 recognizes the sequence and conformation around the initiation site in mRNA, while the above-mentioned proteins may unwind secondary structure in mRNA and anchor it at the 5 -terminal cap structure. 

The absence of 5 -proximal sequences in both eIF-2 and. ribosome-protected segments of mengovirus RNA (Perez-Bercoff and Kaempfer, 1982) supports the concept that the RNA of this virus has evolved a highly efficient mechanism of initiation that bypasses the need for either a 5 end or a 5 -terminal cap structure. Conceivably,... 

Banerjee, A. K., 1980,5 -Terminal cap structure in eucaryotic messenger ribonucleic acids, Microbiol. Rev. 44 175. 

Met-tRNAfMct first binds to the small subunit as a ternary complex with the eIF-2 and GTP. This ternary complex then binds to the 5 end of mRNA with the aid of several factors, one of which, eIF-4F, contains a subunit that specifically binds to the terminal cap structure of the mRNA. Binding to mRNA is followed by a scanning reaction that moves the small subunit along the mRNA, usually to the first AUG, in a reaction driven by the hydrolysis of ATP to ADP and Pj. 

Figure 35-10. The cap structure attached to the 5 terminal of most eukaryotic messenger RNA molecules. A 7-methylguanosine triphosphate (black) is attached at the 5 terminal of the mRNA (shown in blue), which usually contains a 2 -0-methylpurine nucleotide.

As mentioned above, mammahan mRNA molecules contain a 7-methylguanosine cap structure at their 5 terminal, and most have a poly(A) tail at the 3 terminal. The cap stmcmre is added to the 5 end of the newly transcribed mRNA precursor in the nucleus prior to transport of the mELNA molecule to the cytoplasm. The S cap of the RNA transcript is required both for efficient translation initiation and protection of the S end of mRNA from attack by S —> S exonucleases. The secondary methylations of mRNA molecules, those on the 2 -hydroxy and the N of adenylyl residues, occur after the mRNA molecule has appeared in the cytoplasm. 

The first processing event (Eq. 28-6) for most of the pre-mRNA and snRNA transcripts made by RNA polymerase II is addition to the 5 end of a "cap," a terminal structure containing 7-methylguanosine from which a proton has dissociated to form a dipolar ion.563 565 The cap structure may be abbreviated 5 -m7G(5 )pppNm —. The 5 terminal ribose is often methylated on 02 , as shown below. More complex caps are methylated at additional sites, e.g., the guanine may be dimethylated on the 2-NH2 group.551 Most snRNAs, including the U1-U5 and U7-U13 snRNAs, have such 2,2,7-trimethylguanosine... 

Structure of the 5 methylated cap of eukaryotic mRNA. A 7-methylguanosine (in red) is attached through a triphosphate linkage formed between its 5 -OH and the 5 -OH of the terminal residue in... 

Traditionally, a splicing construct contains an intron flanked by two exons. However, interactions across the exons play an important role in spliceosome assembly, a phenomenon termed exon definition.6 The presence of an upstream 3 -splice site generally stimulates the function of a weak downstream 5 -splice site. Similarly, a downstream 5 -splice site induces the usage of a weak upstream 3 -splice site. In this transactivation model, the 5 -cap structure of the first exon and the polyadenylation site in the terminal exons act as 3 - and 5 -splice sites, respectively, in promoting the spliceosome assembly. This view is reinforced by the observation that less efficiently spliced introns may be stimulated by moving the 5 -cap closer to the 5 -splice site or by inserting a functional 5 -splice- or polyadenylation site in the 3 -exon. However, exon definition seems only to play a minor role for efficiently spliced introns in vitro. 

The 3D structural analyses of firefly luciferase (apo form, 2.0 A resolution), " gramicidin S synthetase 1 (PheA), complexed with AMP (107) and L-Phe (1) (1.9A resolution), " " as well as Acs complexed with adenosine-5 -propylphosphate (109) and CoA (106) (1.75 A resolution), " have established the presence of a two-domain architecture. In each case, a comparatively large N-terminal core domain is connected to a much smaller C-terminal cap domain by a solvated peptide linker, with the active site situated at the interface between the two domains. In addition, 3D structures of CBL " as a binary complex with 4-chlorobenzoyl-adenylate (4-CB-AMP) (111, Figure 23) and as a ternary complex with 4-chlorophenacyl-CoA (113) (aninert... 

At the end of polymerization, the 5 end of the pre-mRNA molecule is modified by addition of a N-7-methyl guanine molecule (Fig. 6.15). The 5 terminal phosphate is removed by a phosphatase, and the resulting diphosphate 5 end reacts with the a-phosphate of GTP to form an unusual 5, 5 -triphosphate link (Cap 0). The 5 -cap also may be methylated by S-adenosylmethionine on the 2 -0H of the ribose sugar of the adjacent nucleotide (Cap 1) or on both ribose sugars in the 2 and 3 positions (Cap 2) (Fig. 6.15). This cap structure protects the 5 end of the primary transcript against attack by ribonucleases... 

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