Scanning tunneling microscopy ofawheatseedstorage protein reveals details ofanunusual supersecondary structure

Scanning tunneling microscopy hasbeenused todemonstrate that aspiral structure based on-reverse turns isadopted bytherepeat sequences present inagroup ofwheat gluten proteins. This structure issmilar tothe«spiral formed byasynthetic polypentapeptide based onarepeat sequence present inelastin. Wheatgluten andelastin arebothelasto- meric anditispossible that thespiral structure contributes to this property. Scanning tunneling microscopy (STM)andthederivative scanning probe techniques canproduce high-resolution im- ages ofstructures attheatomic andmolecular levels. Already usedasatool insurface science (1), manyoftheproperties ofSTMhavegreat potential forthestudy ofbiopolymers. TheSTMcanoperate inairandeveninliquid toimage uncoated andunstained biomolecules deposited onacon- ducting surface. Thisallows biopolymers intheir native hydrated state tobeimaged (2-5). STMimages ofDNA have confirmed thedetails ofthehelical structure established by x-ray diffraction, giving confidence inthis formofmicros- copy(6-10). STMcan, therefore, beusedtoimage structures that, toourknowledge, havenotbeendescribed byother techniques. Inthepresent study STMhasbeenusedtostudy thestructure ofahighmolecular weight (HMW)subunit protein fromwheat gluten forwhich anunusual structure has beenpredicted fromtheamino acid sequence andonthebasis ofother physicochemical studies. TheHMW subunits ofwheatgluten appear tobelargely responsible fortheelastic behavior ofdough. Analyses of genomic clones encoding several subunits haveshownthat they havesimilar structures (11, 12). Eachprotein consists of acentral repetitive domain, varying inlength fromabout 640 to830residues, flanked byshorter nonrepetitive N (81-104 residues)- andC(42residues)-terminal domains. TheHMW subunits areclassified into twogroups onthebasis oftheir molecular weights, x-types (molecular weights intherange 83,000-88,000) andy-types (molecular weights intherange 67,000-74,000). Thecentral repetitive domains arebased on threemotifs. Hexapeptides (consensus Pro-Gly-Gln-Gly- Gln-Gln) andnonapeptides (consensus Gly-Tyr-Tyr-Pro- Thr- Ser-Pro/Leu-Gln-Gln)