Accumulation of maize gamma-zein and gamma-zein:KDEL to high levels in tobacco leaves and differential increase of BiP synthesis in transformants

Two gene constructs (pROK.TG1L and pROK.TG1LK) were utilized to achieve accumulation of maize ?-zein to high levels in tobacco (Nicotiana tabacum L.) leaves. Both the chimaeric genes contained the ?- zein-coding region preceded by the 5?untranslated leader from the coat protein mRNA of TMV, but one of them (pROK.TG1LK) was modified in its protein-coding region by the addition of the ER retention signal KDEL. The accumulation of ?-zein and ?-zein:KDEL in leaves was compared with heterologous protein accumulation in tobacco plants previously transformed with a ?-zein cDNA harbouring a native 5?UTR. Replacement of ?-zein 5?UTR with the TMV leader dramatically increased ?- zein production. Furthermore, ?-zein:KDEL-expressing plants, on average, accumulated twice as much foreign protein in their leaves as pROK.TG1L plants. The twofold increase in the level of ?-zein:KDEL can probably be attributed to an improvement in the mechanism for ER retention of zeins in the transgenic cells. Transformants also showed increased production of BiP, though to a lesser extent in ?-zein:KDEL-expressing plants compared with pROK.TG1L plants. It is therefore likely that ?- zein:KDEL retention is made less dependent on the chaperone assistance of BiP by the presence of the KDEL signal on the ?-zein mutant.