The cap structure and the poly(A) tail of eukaryotic mRNAs act synergistically to enhance translation. This effect is mediated by a direct interaction of eukaryotic initiation factor 4G and poly (

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The C-terminal domain of poly(A)-binding protein (PABC) is a peptide-binding domain found in poly(A)-binding proteins (PABPs) and a HECT (homologous to E6-AP Cterminus) family E3 ubiquitin ligase. In protein synthesis, the PABC domain of PABP functions to recruit several translation factors possessing the PABP-interacting motif 2 (PAM2) to the mRNA poly(A) tail. We have determined the solution structure of the human PABC domain in complex with two peptides from PABP-interacting protein-1 (Paip1) and Paip2. The structures show a novel mode of peptide recognition, in which the peptide binds as a pair of b-turns with extensive hydrophobic, electrostatic and aromatic stacking interactions. Mutagenesis of PABC and peptide residues was used to identify key protein-peptide interactions and quantified by isothermal calorimetry, surface plasmon resonance and GST pulldown assays. The results provide insight into the specificity of PABC in mediating PABP-protein interactions.

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We have de novo designed a heterodimeric coiled-coil formed by two peptides as a capture/ delivery system that can be used in applications such as affinity tag purification, immobilization in biosensors, etc. The two strands are designated as K coil (KVSALKE heptad sequence) and E coil (EVSALEK heptad sequence), where positively charged or negatively charged residues occupy positions e and g of the heptad repeat. In this study, for each E coil or K coil, three peptides were synthesized with lengths varying from three to five heptads. The effect of the chain length of each partner upon the kinetic and thermodynamic constants of interaction were determined using a surface plasmon resonance-based biosensor. Global fitting of the interactions revealed that the E5 coil interacted with the K5 coil according to a simple binding model. All the other interactions involving shorter coils were better described by a more complex kinetic model involving a rate-limiting reorganization of the coiled-coil structure. The affinities of these de novo designed coiled-coil interactions were found to range from 60 pM (E5/K5) to 30 µM (E3/K3). From these K d values, we were able to determine the free energy contribution of each heptad, depending on its relative position within the coiled-coils. We found that the free energy contribution of a heptad occupying a central position was 3-fold higher than that of a heptad at either end of the coiled-coil. The wide range of stabilities and affinities for the E/K coil system provides considerable flexibility for protein engineering and biotechnological applications.The R-helical coiled-coil is an oligomerization domain that is naturally present in a wide variety of proteins such as transcription factors, cytoskeletal proteins, motor proteins, and viral fusion proteins (1-3). The structural properties of coiled-coils have been extensively characterized from numerous high-resolution crystal and NMR structures (3-5). The formation of coiled-coils requires the wrapping of two or more amphipathic R-helices around each other in a lefthanded supercoil fashion; the helices may be aligned in either a parallel or antiparallel manner. The R-helices composing the coiled-coil structure are defined by a heptad repeat (denoted abcdefg) in which hydrophobic residues occupy the positions a and d and pack in a characteristic "knobs-intoholes" manner (6), forming the hydrophobic core ( Figure 1). Many studies (1,(7)(8)(9)(10)(11)(12) emphasized the role of charged residues (typically in the e and g positions) in controlling the specificity of oligomerization and opened up the way to the de novo design of heterodimeric coiled-coils (13-17). Their relatively small size, in addition to their well-defined structure, make coiled-coils a very attractive system for protein engineering and biotechnological applications (5, 18) such as the construction of miniaturized antibodies, the control of signaling protein domain oligomerization, and the specific targeting of GFP to cytoskeletal proteins (see ref 4 for ...