Scanning tunneling microscopy has been used to demonstrate that a spiral structure based on -reverse turns is adopted by the repeat sequences present in a group of wheat gluten proteins. This structure is smilar to the «spiral formed by a synthetic polypentapeptide based on a repeat sequence present in elastin. Wheat gluten and elastin are both elastomeric and it is possible that the spiral structure contributes to this property.Scanning tunneling microscopy (STM) and the derivative scanning probe techniques can produce high-resolution images of structures at the atomic and molecular levels. Already used as a tool in surface science (1), many of the properties of STM have great potential for the study of biopolymers. The STM can operate in air and even in liquid to image uncoated and unstained biomolecules deposited on a conducting surface. This allows biopolymers in their native hydrated state to be imaged (2-5). STM images of DNA have confirmed the details of the helical structure established by x-ray diffraction, giving confidence in this form of microscopy (6-10). STM can, therefore, be used to image structures that, to our knowledge, have not been described by other techniques. In the present study STM has been used to study the structure of a high molecular weight (HMW) subunit protein from wheat gluten for which an unusual structure has been predicted from the amino acid sequence and on the basis of other physicochemical studies.The HMW subunits of wheat gluten appear to be largely responsible for the elastic behavior of dough. Analyses of genomic clones encoding several subunits have shown that they have similar structures (11,12). Each protein consists of a central repetitive domain, varying in length from about 640 to 830 residues, flanked by shorter nonrepetitive N (81-104 residues)-and C (42 residues)-terminal domains. The HMW subunits are classified into two groups on the basis of their molecular weights, x-types (molecular weights in the range 83,000-88,000) and y-types (molecular weights in the range 67,000-74,000). The central repetitive domains are based on three motifs. Hexapeptides (consensus Pro-Gly-Gln-GlyGln-Gln) and nonapeptides (consensus Gly-Tyr-Tyr-Pro-ThrSer-Pro/Leu-Gln-Gln) are present in both x-and y-type subunits and tripeptides (consensus Gly-Gln-Gln) in x-type subunits only. The central domains are predicted to form regularly repeated (-reverse turns (13). Further evidence for the presence of (-reverse turns has come from spectroscopic (circular dichroism and Fourier-transform infrared) studies of synthetic peptides corresponding to the repeat motifs present in both x-and y-type subunits (14). Hydrodynamic studies of a single purified x-type HMW subunit from durum wheat showed an extended rod-like conformation and it was proposed that the (-reverse turns form a loose spiral (15). Modeling of several proteins containing (-reverse turns has indicated the possibility of spiral structures (16), and detailed studies of a synthetic polypentapeptide based on the repeat motif of e...

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Crystals formed from a mixture of tropomyosin and troponin T have an open double-stranded lattice structure with a diamond-shaped repeat. In some regions the appearance in electron micrographs of negatively stained specimens changes from this double-diamond lattice to a more condensed banded crystal form. The double-diamond lattice has plane group symmetry cmm with unit cell 76.3 by 21.7 nm. The molecules form continuous chains along the diagonal of the unit cell and the diagonal length (79.4 nm) is that expected for two tropomyosin molecules joined end-to-end. Computer filtering of the micrographs shows that the strands of the lattice are thicker from the acute vertex of the large diamond to a point about half-way along the side of the diamond, where there is a small blob of density. At the acute vertex of the diamond is a large blob of density which is accentuated, however, by being at the lattice node where strands cross each other, and which is much weaker in regions of the micrographs where the crystals have condensed laterally. The results indicate that troponin T is a long thin molecule running in contact with the tropomyosin strands over 40-50% of the tropomyosin molecular length. The small globular region may represent the end-to-end overlap of tropomyosin but is more likely to be a globular region at the C-terminal region of troponin T.