Six integral membrane proteins of bacterial, animal, and plant origin, which are believed to function in solute transport, share sequence identity and are grouped together as members of the MIP family. These include the Escherichia coli glycerol facilitator, the major intrinsic protein from bovine lens fibre junction membranes, a plant tonoplast membrane protein, a soybean protein from the peribacteroid membrane, and a Drosophila neurogenic protein. These proteins, each of which appears to consist of six transmembrane helical segments per subunit, apparently arose by internal duplication of a three-transmembrane segment. Phylogenetic 'trees' interrelating these proteins and segments are presented.
Addition of gibberellic acid to barley endosperm evokes the formation of a-amylase and of other hydrolases. Protein synthesis inhibitors -notably cycloheximide -inhibit the production of the hydrolases and the incorporation of labeled amino acids into proteins. Isolation, purification, and "fingerprinting" of the gibberellic acid-induced a-amylase indicate that it is formed by de novo synthesis. RNA synthesis inhibitors -notably actinomycin D -also inhibit the production of a-amylase and of the other hydrolases. Gibberellic acid enhances the incorporation of labeled RNA precursors into RNA. These data are consistent with the idea that gibberellic acid controls the level of a-amylase and of other enzymes by controlling the synthesis of specific RNA's which in turn control the synthesis of specific enzymes.
Cotyledons of developing pea seeds (Pisurn sativum L.) were labeled with radioactive amino acids and glucosamine, and extracts were prepared and separated into fractions rich in endoplasmic reticulum (ER) or protein bodies . The time-course of synthesis of the polypeptides of legumin and vicilin and the site of their assembly into protein oligomers were studied using immunoaffinity gels and sucrose density gradients. When cotyledons were pulselabeled (1-2 h), newly synthesized legumin was present in polypeptides with Mr 60,000-65,000, and newly synthesized vicilin was present as a series of polypeptides with Mr 75,000, 70,000, 50,000, and 49,000. These radioactive polypeptides were found primarily in the ER (Chrispeels et al ., 1982, /. Cell BioL, 93 :5-14) . During a subsequent chase period, newly synthesized reserve proteins were initially present in the protein bodies in the above-named polypeptides . Between 1 and 20 h later, radioactive legumin subunits (M r 40,000 and 19,000) and smaller vicilin polypeptides (Mr 34,000, 30,000, 25,000, 18,000, 14,000, 13,000, and 12,000) appeared in the protein bodies . The appearance of these labeled polypeptides in the protein bodies was not the result of a slow transport from the ER (or cytoplasm) .Newly synthesized legumin and vicilin polypeptides were assembled into oligomers of 85 and 7S, respectively, in the ER . They appeared in the protein bodies in these oligomeric forms before the appearance of the smaller polypeptides (M r <49,000) . These results indicate that the smaller vicilin polypeptides (M r <49,000) arise by delayed posttranslational processing of some or all of the larger vicilin polypeptides . The precursors of legumin are completely processed in the protein bodies 2-3 h after their synthesis. The processing of the vicilin precursors is much slower (6-20 h) and only a fraction of the precursor molecules are processed . As a result both large (M r >49,000) and small polypeptides of vicilin accumulate in the protein bodies, whereas legumin accumulates only as polypeptides of Mr 40,000 and 19,000 .The large parenchymal cells of the cotyledons of pea (Pisum sativum L.) seeds contain 20-30% protein on a dry weight basis. The reserve proteins vicilin and legumin make up 70% of this protein. These reserve proteins are localized in protein bodies, which are spherical organelles measuring 1-3 gm in diameter consisting of an amorphous protein matrix surrounded by a limiting membrane. In mature seeds, legumin is a 12S protein (Mr 360,000) that consists of six acidic (Mr 40,000) and six basic (Mr 20,000) subunits, linked together in pairs by disulfide bridges (11). When legumin is fractionated by SDS PAGE, heterogeneity is found in both the acidic and basic subunits with three to five polypeptides in each molecular weight class .306
The membranes of plant and animal cells contain aquaporins, proteins that facilitate the transport of water. In plants, aquaporins are found in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1, a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-ProAla motif. Here, we report the molecular analysis of a new Arabidopsis aquaporin, 6-TIP (for tonoplast intrinsic protein), and show that it is located in the tonoplast. The water channel activity of 6-TIP is sensitive to mercury. However, the mercurysensitive cysteine residue found in mammalian aquaporins is not present in 6-TIP or in 1-TIP, a previously characterized mercury-sensitive tonoplast aquaporin. Site-directed mutagenesis was used to identify the mercury-sensitive site in these two aquaporins as Cys-116 and Cys-118 for 6-TlP and y-TIP, respectively. These mutations are at a conserved position in a presumed membrane-spanning domain not previously known to have a role in aquaporin mercury sensitivity. Comparing the tissue expression patterns of 651P with y5IP and a-TIP showed that the TlPs are differentially expressed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.