Expression of maize gamma zein C-terminus in Escherichia coli

Prolamins containing a highly conserved cysteine-rich C-terminal domain have been poorly expressed as soluble protein in model systems such as Escherichia coli. Possible reasons have included a combination of the reducing environment of the bacterial cytoplasm and protein secondary structure. Using a bacterial thioredoxin fusion expression system, full-length native gamma zein, native gamma zein C-terminus, and modified gamma zein C-terminus, containing 13 amino acid changes, were found to accumulate up to 58, 50, and 42% of the total cellular protein, respectively. The native gamma zein C-terminus fusion protein was six times more soluble (70%) than the full-length fusion protein (12%), four times more soluble than the N-terminus (19%), and eight times more soluble than the modified C-terminus (9%). The modified C-terminal domain contained amino acid changes that improved the lysine, isoleucine, and tryptophan content, while removing two evolutionarily conserved cysteines and one nonconserved cysteine. Expression of the native C-terminal domain without thioredoxin resulted in decreased solubility (13%) and decreased expression (8%). In contrast, coexpression with thioredoxin resulted in a sevenfold increase in solubility (86%). These results suggest that insolubility of full-length gamma zein results from structural interactions of the N-terminus and that solubility of the C-terminal domain is dependent on proper disulfide bond formation. The ability to express the C-terminal domain of gamma zein as soluble protein should allow future identification of important structural elements in gamma zein and similar proteins.