Mechanisms of assembly of wheat high molecular weight glutenins inferred from expression of wild-type and mutant subunits in transgenic tobacco

Following sequestration into the endoplasmic reticulum, wheat high molecular weight glutenin subunits (HMW-GS) assemble into polymers through intermolecular disulfide bond formation. These polymers, which also include low molecular weight glutenin subunits (LMW-GS), have a broad distribution of molecular mass reaching up to several million daltons. To study the mechanism of assembly of the HMW-GS, we have expressed x- and y-type HMW-GS in transgenic tobacco plants. Both types, when expressed individually or in combination, were incorporated into polymers. Partial reduction of polymers formed by different subunits resulted in different patterns of release of homodimers, heterodimers, and monomers. This suggested different arrangements of intermolecular disulfide bonds or different peptide conformations in the vicinity of the disulfide bonds linking x-x, y-y, and x-y type HMW-GS. A mutant of the x-type subunit, lacking a conserved cysteine in the C-terminal domain, assembled into oligomers linked by intermolecular disulfide bonds, but not into large polymers. This mutant was deposited, however, in dense protein bodies, similar to those formed by the native HMW-GS, suggesting that polymer formation and packaging into protein bodies may be the result of different types of interactions. Pulse-chase labeling of proteins in wheat endosperm showed that the assembly of the HMW-GS into insoluble polymers occurs by a slow process which apparently continues after the initiation of protein body formation.