Adenosine (Ado) kinase (ADK; ATP:Ado 5Ј phosphotransferase, EC 2.7.1.20) catalyzes the salvage synthesis of adenine monophosphate from Ado and ATP. In Arabidopsis, ADK is encoded by two cDNAs that share 89% nucleotide identity and are constitutively, yet differentially, expressed in leaves, stems, roots, and flowers. To investigate the role of ADK in plant metabolism, lines deficient in this enzyme activity have been created by sense and antisense expression of the ADK1 cDNA. The levels of ADK activity in these lines range from 7% to 70% of the activity found in wild-type Arabidopsis. Transgenic plants with 50% or more of the wild-type activity have a normal morphology. In contrast, plants with less than 10% ADK activity are small with rounded, wavy leaves and a compact, bushy appearance. Because of the lack of elongation of the primary shoot, the siliques extend in a cluster from the rosette. Fertility is decreased because the stamen filaments do not elongate normally; hypocotyl and root elongation are reduced also. The hydrolysis of S-adenosyl-l-homo-cysteine (SAH) produced from S-adenosyl-l-methionine (SAM)-dependent methylation reactions is a key source of Ado in plants. The lack of Ado salvage in the ADK-deficient lines leads to an increase in the SAH level and results in the inhibition of SAMdependent transmethylation. There is a direct correlation between ADK activity and the level of methylesterified pectin in seed mucilage, as monitored by staining with ruthenium red, immunofluorescence labeling, or direct assay. These results indicate that Ado must be steadily removed by ADK to prevent feedback inhibition of SAH hydrolase and maintain SAM utilization and recycling.

In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1-5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.Fertilization of the angiosperm ovule not only results in the development of the embryo and endosperm, but also initiates differentiation of the ovule integuments to form the seed coat. The seed coat consists of multiple specialized cell layers that play important roles in embryo protection and the regulation of germination. One specialization is known as myxospermy, a property of epidermal cells whereby they produce large quantities of pectic polysaccharide (mucilage; Frey-Wyssling, 1976; Grubert, 1981; Boesewinkel and Bouman, 1995). Myxospermy is commonly found in species of the Brassicaceae, Solanaceae, Linaceae, and Plantaginaceae, where mucilage forms a gel-like capsule surrounding the seed upon imbibition. Proposed roles for mucilage include facilitating seed hydration and/or dispersal. Mucilages are also found in the root cap and transmitting tract (Frey-Wyssling, 1976; Esau, 1977), where they foster root tip and pollen tube growth, respectively.Mucilages are largely composed of pectins, a heterogeneous group of complex, acidic polysaccharides that also comprise the majority of the plant cell wall matrix. Dicotyledonous pectins largely consist of poly-GalUA (PGA) and rhamnogalacturonan I (RG I; Brett and Waldron, 1990; Carpita and Gibeaut, 1993; Cosgrove, 1997). PGA is composed of an unbranched chain of ␣1,4-linked GalUA residues, whereas RG I is a highly branched polysaccharide with a backbone of alternating ␣1,4-linked GalUA and ␣1,2-linked rhamnose (Rha), with sugar side chains attached to the Rha residues (Brett and Waldron, 1990). The degree of gelling of pectins is largely dependent on ionic bonding between PGA molecules and free divalent calcium. Thus, cell wall fluidity is affected by the degree of methyl esterification of...

Heterotrimeric G protein alpha subunits, RGS proteins, and GoLoco motif proteins have been recently implicated in the control of mitotic spindle dynamics in C. elegans and D. melanogaster. Here we show that "regulator of G protein signaling-14" (RGS14) is expressed by the mouse embryonic genome immediately prior to the first mitosis, where it colocalizes with the anastral mitotic apparatus of the mouse zygote. Loss of Rgs14 expression in the mouse zygote results in cytofragmentation and failure to progress to the 2-cell stage. RGS14 is found in all tissues and segregates to the nucleus in interphase and to the mitotic spindle and centrioles during mitosis. Alteration of RGS14 levels in exponentially proliferating cells leads to cell growth arrest. Our results indicate that RGS14 is one of the earliest essential product of the mammalian embryonic genome yet described and has a general role in mitosis.

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Regulators of G-protein Signaling (RGS proteins)

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