An overview of CD of proline-rich peptides is reported. First, structural characteristics, theoretical CD studies, and the biological relevance of polyproline II structure in such peptides are discussed. Second, a CD study of peptides belonging to the repetitive domain of maize glutelin-2, H-(Val-His-Leu-Pro-Pro-Pro)n-OH (n = 3, 5, 8), is described. This series of peptides displayed the CD features of polyproline II structure in water (5 degrees C, pH 5). Moreover, it was shown that the addition of increasing amounts of the polyanionic molecule heparin forced a displacement of the conformational equilibrium of those peptides toward higher proportions of the polyproline II structure. In contrast, when the temperature is raised such a structure gradually disappears, leading to more disordered conformations.
Background: Protein bodies (PBs) are natural endoplasmic reticulum (ER) or vacuole plantderived organelles that stably accumulate large amounts of storage proteins in seeds. The prolinerich N-terminal domain derived from the maize storage protein γ zein (Zera) is sufficient to induce PBs in non-seed tissues of Arabidopsis and tobacco. This Zera property opens up new routes for high-level accumulation of recombinant proteins by fusion of Zera with proteins of interest. In this work we extend the advantageous properties of plant seed PBs to recombinant protein production in useful non-plant eukaryotic hosts including cultured fungal, mammalian and insect cells.
Reports from a number of laboratories describe the presence of a family of proteins (the major intrinsic protein family) in a variety of organisms. These proteins are postulated to form channels that function in metabolite transport. In plants, this family is represented by the product of NOD26, a nodulation gene in soybean that encodes a protein of the peribacteroid membrane, and tonoplast intrinsic protein (TIP), an abundant protein in the tonoplast of protein storage vacuoles of bean seeds (KD Johnson, H Hofte, MJ Chrispeels [1990] Plant Cell 2: 525-532). Other homologs that are induced by water stress in pea and in Arabidopsis thaliana and that are expressed in the roots of tobacco have been reported, but the location of the proteins they encode is not known. We now report the presence and derived amino acid sequences of two different TIP proteins in A. thaliana. a-TIP is a seed-specific protein that has 68% amino acid sequence identity with bean seed TIP; 'y-TIP is expressed in the entire vegetative body of A. thaliana and has 58% amino acid identity with bean seed TIP. Both proteins are associated with the tonoplast. Comparsons of the derived amino acid sequences of the seven known plant proteins in the major intrinsic protein family show that genes with similar expression pattems (e.g. water stress-induced or seed specific) are more closely related to each other than the three A. thaliana homologs are related. We propose that the nonoverlapping gene expression patterns reported here, and the evolutionary relationships indicated by the phylogenetic tree, suggest a functional specialization of these proteins. diated by carriers, allowing fluxes of solutes against the electrochemical gradient and channels that permit passive transport, i.e. down the electrochemical gradient. Because the vacuoles of different cell types can show dramatic differences in the nature and magnitude of the ion and metabolite exchange with the cytoplasm (e.g. transport of potassium in guard cells or of malate in the mesophyll of CAM plants), the tonoplasts most likely have different channels and carriers with specialized functions. Recently, published reports point to a small, but growing, family of homologous intrinsic membrane proteins from quite different organisms (22,28). This family has been termed the MIP4 family after its best characterized member, MIP from bovine lens fiber junctional membranes. MIP has been shown to form tetrameric structures (1) with channel activity in artificial lipid membranes (7). One of the MIP homologs, GLPF, in the inner membrane of Escherichia coli plays a role in the facilitated transport of glycerol. By analogy, it seems reasonable to postulate that the other proteins in this superfamily also form channels that are involved in passive transmembrane transport of ions and/or metabolites. The first member of this family to be identified in plants was NOD26 (24), a plant-encoded protein in the peribacteroid membrane of soybean root nodules infected with Rhizobium bacteroids. Recently, three ...
y-Zein is a maize storage protein synthesized by endosperm cells and stored together with a-and P-zeins in specialized organelles called protein bodies. Previous studies have shown that in maize there is only one type of protein body and it is derived directly from the endoplasmic reticulum (ER). In this article, we describe the domains of y-zein involved in ER retention and the domains involved in protein body formation. To identify the signal responsible for y-zein retention in ER-derived protein bodies, DNAs encoding various deletion mutants of y-zein were constructed and introduced into Arabidopsis as a heterologous system. By using pulse-chase experiments and immunoelectron microscopy, we demonstrated that the deletion of a proline-rich domain at the N terminus of y-zein puts an end to its retention in the ER; this resulted in the secretion of the mutated protein. The amino acid sequence of y-zein necessary for ER retention is the repeat domain composed of eight units of the hexapeptide PPPVHL. In addition, we obsenred that only those y-zein mutants that contained both the proline-rich repeat domain and the C-terminal cysteine-rich domain were able to form ER-derived protein bodies. We suggest that the retention of y-zein in the ER could be a result of a protein-protein association or a transient interaction of the repeat domain with ER membranes.
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