Scanning tunneling microscopy of a wheat seed storage protein reveals details of an unusual supersecondary structure.

Abstract

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...

Structure characterization of the central repetitive domain of high molecular weight gluten proteins. II. Characterization in solution and in the dry state

“…The dry repetitive domain probably consists of a continuous series of these turns, which could organize into a so-called P-spiral, similar to that of elastin (Urry, 1993). The presence of this unusual supersecondary structure has been suggested previously from different studies (e.g., Tatham et al, 1984;Miles et al, 1991). Our hypothesis of the conversion of type I to type I1 p-turns is compatible with these observations.…”

“…Its structural organization may consist of a series of p-turns that organize in a @spiral structure (Tatham et al, 1984;Shewry et al, 1994); this idea is supported by scanning tunneling microscopy images of HMW Dx5 (Miles et al, 1991). The proposed &spiral structure for HMW proteins is similar to that of elastin, where it forms the basis for the molecule's elastic properties (Uny, 1993).…”

The structure and properties of gluten: an elastic protein from wheat grain

“…It has also been proposed that the β-turns are organized to give a regular spiral structure (termed a β-spiral) similar to that demonstrated for a synthetic polypentapeptide based on a repeat motif of elastin (Urry 1988). Molecular modelling can be used to generate such spiral structures (figure 4) whose dimensions (diameter, pitch and length) are consistent with those determined by viscometric analysis and revealed by STM of purified proteins in the hydrated solid state (Miles et al 1991). However, Kasarda et al (1994) have proposed that an alternative type of spiral structure is formed, based on γ-turns rather than β-turns.…”

Comparative structures and properties of elastic proteins

“…Hydrodynamic studies indicated a rod-like structure in solution (Field et al 1987) and STM images showed that the central repetitive domains adopt a helical structure with a diameter of ca.1.9 nm and a pitch of ca. 1.5 nm in the hydrated solid state (Miles et al 1991). Thus, it has been proposed that the central repetitive domains form a β-spiral type structure.…”