The structure of the ternary complex consisting of yeast phenylalanyl-transfer RNA (Phe-tRNAPhe), Thermus aquaticus elongation factor Tu (EF-Tu), and the guanosine triphosphate (GTP) analog GDPNP was determined by x-ray crystallography at 2.7 angstrom resolution. The ternary complex participates in placing the amino acids in their correct order when messenger RNA is translated into a protein sequence on the ribosome. The EF-Tu-GDPNP component binds to one side of the acceptor helix of Phe-tRNAPhe involving all three domains of EF-Tu. Binding sites for the phenylalanylated CCA end and the phosphorylated 5' end are located at domain interfaces, whereas the T stem interacts with the surface of the beta-barrel domain 3. The binding involves many conserved residues in EF-Tu. The overall shape of the ternary complex is similar to that of the translocation factor, EF-G-GDP, and this suggests a novel mechanism involving "molecular mimicry" in the translational apparatus.

The crystal structure of intact elongation factor Tu (EF-Tu) from Thermus thermophilus has been determined and refined at an effective resolution of 1.7 A, with incorporation of data extending to 1.45 A. The effector region, including interaction sites for the ribosome and for transfer RNA, is well defined. Molecular mechanisms are proposed for transduction and amplification of the signal induced by GTP binding as well as for the intrinsic and effector-enhanced GTPase activity of EF-Tu. Comparison of the structure with that of EF-Tu-GDP reveals major mutual rearrangements of the three domains of the molecule.

The mode of interactions with tRNA explains the absolute necessity for the (alphabeta)2 architecture of PheRS. The interactions of tRNAPhe with PheRS and particularly with the coiled-coil domain of the alpha subunit result in conformational changes in TPsiC and D loops seen by comparison with uncomplexed yeast tRNAPhe. The tRNAPhe is a newly recognized type of RNA molecule specifically interacting with the RBD fold. In addition, a new type of anticodon-binding domain emerges in the aaRS family. The uniqueness of PheRS in charging 2'OH of tRNA is dictated by the size of its adenine-binding pocket and by the local conformation of the tRNA's CCA end.

The crystal structure of phenylalanyl-tRNA synthetase from Thermus thermophilus, solved at 2.9 A resolution, displays (alpha beta)2 subunit organization. Unexpectedly, both the catalytic alpha- and the non-catalytic beta-subunits comprise the characteristic fold of the class II active-site domains. The alpha beta heterodimer contains most of the building blocks so far identified in the class II synthetases. The presence of an RNA-binding domain, similar to that of the U1A spliceosomal protein, in the beta-subunit is indicative of structural relationships among different families of RNA-binding proteins. The structure suggests a plausible catalytic mechanism which explains why the primary site of tRNA aminoacylation is different from that of the other class II enzymes.

Unlike processive cellular motors such as myosin V, whose structure has recently been determined in a "rigor-like" conformation, myosin II from contracting muscle filaments necessarily spends most of its time detached from actin. By using squid and sea scallop sources, however, we have now obtained similar rigor-like atomic structures for muscle myosin heads (S1). The significance of the hallmark closed actin-binding cleft in these crystal structures is supported here by actin/S1-binding studies. These structures reveal how different duty ratios, and hence cellular functions, of the myosin isoforms may be accounted for, in part, on the basis of detailed differences in interdomain contacts. Moreover, the rigor-like position of switch II turns out to be unique for myosin V. The overall arrangements of subdomains in the motor are relatively conserved in each of the known contractile states, and we explore qualitatively the energetics of these states.

Tropomyosin is a two-chain ␣-helical coiled coil whose periodic interactions with the F-actin helix are critical for thin filament stabilization and the regulation of muscle contraction. Here we deduce the mechanical and chemical basis of these interactions from the 2.3-Å-resolution crystal structure of the middle three of tropomyosin's seven periods. Geometrically specific bends of the coiled coil, produced by clusters of core alanines, and variable bends about gaps in the core, produced by isolated alanines, occur along the molecule. The crystal packing is notable in signifying that the functionally important fifth period includes an especially favorable protein-binding site, comprising an unusual apolar patch on the surface together with surrounding charged residues. Based on these and other results, we have constructed a specific model of the thin filament, with the N-terminal halves of each period (i.e., the so-called ''␣ zones'') of tropomyosin axially aligned with subdomain 3 of each monomer in F-actin.alanine ͉ ␣-helix ͉ cardiomyopathy ͉ coiled coil ͉ packing

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Contraction in striated and cardiac muscles is regulated by the motions of a Ca 2؉ -sensitive tropomyosin͞troponin switch. In contrast, troponin is absent in other muscle types and in nonmuscle cells, and actomyosin regulation is myosin-linked. Here we report an unusual crystal structure at 2.7 Å of the C-terminal 31 residues of rat striated-muscle ␣-tropomyosin (preceded by a fragment of the GCN4 leucine zipper). The C-terminal 22 residues (263-284) of the structure do not form a two-stranded ␣-helical coiled coil as does the rest of the molecule, but here the ␣-helices splay apart and are stabilized by the formation of a tail-to-tail dimer with a symmetry-related molecule. The site of splaying involves a small group of destabilizing core residues that is present only in striated muscle tropomyosin isoforms. These results reveal a specific recognition site for troponin T and clarify the physical basis for the unique regulatory mechanism of striated muscles.