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Definition
γ-Zein -related peptides are present in the repeating region in proline-rich proteins such as corn γ-zein, soybean cell wall protein (SbPRP1), carrot 33-kDa protein, and collagen and mouse salivary gland protein.

Discovery
S.Maruyama et al.,1992 chemically synthesized proline-rich proteins corn γ-zein 1. The prolyl endopeptidase (PEP, from Flavobacterium meningosepticum or bovine brain) inhibitory activity of these peptides was investigated. The N-terminal domain of maize γ-zein has a repetitive structure (Val-His-Leu-Pro-Pro-Pro)8 that has recently been shown to adopt an amphipathic polyproline II type conformation in aqueous solution. Dalcol I et al., in 1996 reported here the synthesis and conformational analysis of three model peptides (Val-X-Leu-Pro-Pro-Pro)8 (X = Ala (1), Glu (2), Lys (3)). The three compounds have been synthesized in a very efficient way using a convergent solid-phase strategy. Circular dichroism shows unequivocally that the three model peptides adopt polyproline II (PPII) type conformations under a variety of experimental conditions and that neither the presence of histidine nor amphipathicity of the peptide is an absolute requirement for adopting the native conformation. These results open the door for the de novo design of compounds with PPII conformations and must be taken into account in the structure prediction of protein structures from sequence data banks 2.

Structural Characteristics
γ-Zein is composed of four characteristic domains i) a peptide signal of 19 amino acids, ii) the repeat domain (53 aa) containing eight units of the hexapeptide PPPVHL, iii) the prox domain where proline residues alternate withother amino acids (29 aa) and iv) the hydrophobic cysteine rich C-terminal domain (111 aa). The ability of γ-zein to assemble in ER-derived PBs is not restricted to seeds. The prolyl endopeptidase (PEP, from Flavobacterium meningosepticum or bovine brain) inhibitory activity of these peptides was investigated 3. γ-Zein-related synthetic peptides such as His-Leu-Pro-Pro-Pro-Val and His-Leu-Pro-Pro-Pro-Val-His-Leu-Pro-Pro-Pro-Val inhibited PEP from both bacterial and meammalian sources. Soybean cel l wall protein-related synthecic peptides, Lys-Pro-Pro-Val and Lys-Pro-Pro-Ile also inhibited PEP to some degree. However, Pro-Pro-Pro-Pro-Gly-Gly-Pro-Glin-Pro-Arg-Pro-Pro-Gln-Gly (synthetic peptide fragment of salivary gland protein) and Gly-Pro-Hyp-Gly-Pro-Ala (fragment of collagen) showed hardly any inhibition. Maruyama S et al.,1992  hydrolyzed γ-zein with sobtilisin (Carlsberg) and confirmed the liberation of the native hexapeptide, His-Leu-Pro-Pro-Pro-Val 1

Mode of Action
Zeins are a group of proteins that are synthesized during endosperm development in corn and may be separated in four groups α, β, γ and δ, based on their solubility. Zeins can aggregate into PB directly in the ER. The ability of γ-zein to assemble in ER-derived PBs is not restricted to seeds. In fact, when γ-zein gene was constitutively expressed in transgenic Arabidopsis plants, the storage protein accumulated within ER-derived PBs in leaf mesophyl cells. Looking for a signal responsible for the γ-zein deposition into the ER-derived PB (prolamins do not have KDEL signal), it has been demonstrated that the proline-rich N-terminal domain including the tandem repeat domain was necessary for ER retention and that the C-terminal domain was involved in PB formation. However, the mechanisms by which these domains promote the PB assembly are still unknown 3. Twenty-seven kilodalton γ-zein is a subclass of the maize zein storage proteins and, due to its localization at the protein body periphery, is critical to digestibility characteristics of all zeins. This protein had low in vitro digestibility, presumably due to its high Cys content (7.35mol%) that is similar to the hard-to-digest analogous sorghum protein, γ-kafirin. Each of the conserved disulfide-bonded Cys’ havebeen mutated to create C144A, C148A, C155A, and C156A maize γ-zein mutants. The C155A showed a remarkable increase in digestibility to proteases – pepsin, chymotrypsin, and trypsin. A high conservation of this Cys among cereal γ-prolamins indicates the utility of this finding 4.

γ-Zein-related synthetic peptides such as His-Leu-Pro-Pro-Pro-Val and His-Leu-Pro-Pro-Pro-Val-His-Leu-Pro-Pro-Pro-Val inhibited PEP from both bacterial and meammalian sources. These two peptides have been shown to inhibit prolylendopeptidase (PEP) from both bacterial and mammalian sources. The IC50 values of H-2254 and H-2256 for Flavobacterium meningosepticum PEP were 80 µM and 30 µM, respectively 1.

Functions
γ-zein expression, when γ-zein gene was constitutively expressed in transgenic Arabidopsis plants, the storage protein accumulated within ER-derived PBs in leaf mesophyl cells 3.

Proline-rich N-terminal domain, looking for a signal responsible for the γ-zein deposition into the ER-derived PB, it has been demonstrated that the proline-rich N-terminal domain including the tandem repeat domain was necessary for ER retention and that the C-terminal domain was involved in PB formation.

γ-Zein, a maize storage protein, is one of the four maize prolamins and represents 10-15% of the total protein in the maize endosperm 5.

Biosynthesis, alpha and gramma-zeins are biosynthesized in membrane-bound polysomes at thecytoplasmic side of the rough ER, assembled within the lumen and then sequestrated into ER-derived PB .

References

1.     Maruyama  S,  Shinsuke  M, Takasumi O,  Hideoki T (1992). Prolyl endopeptidase inhibitory activity of peptides in the repeated sequence of various proline-rich proteins J. Ferment Bioeng., 74:145-148.

2.     Dalcol I, Pons M, Ludevid MD, Giralt E (1996). Convergent Synthesis of Repeating Peptides (Val-X-Leu-Pro-Pro-Pro)8 Adopting a Polyproline II Conformation. J. Org. Chem., 61(20):6775–6782.

3.     Geli MI, Torrent M, Ludevid D (1994). Two Structural Domains Mediate Two Sequential Events in [gamma]-Zein Targeting: Protein Endoplasmic Reticulum Retention and Protein Body Formation. Plant Cell. 6(12):1911-1922.

4.     Lee S, Hamaker B (2009). Cys155 of 27kDa maize γ-zein is a key amino acid to improve its in vitro digestibility. FEBS Letters., 580(25):5803-5806.

5.     Herman EM, Larkins BA (1990). Protein storage bodies and vacuoles Plant Cell., 11(4):601-614.         

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