Heterosis or hybrid vigor is widespread in plants and animals. Although the molecular basis for heterosis has been extensively studied, metabolic and proteomic contributions to heterosis remain elusive. Here we report an integrative analysis of time-series metabolome and proteome data in maize (Zea mays) hybrids and their inbred parents. Many maize metabolites and proteins are diurnally regulated, and many of these show nonadditive abundance in the hybrids, including key enzymes and metabolites involved in carbon assimilation. Compared with robust trait heterosis, metabolic heterosis is relatively mild. Interestingly, most amino acids display negative mid-parent heterosis (MPH), i.e., having lower values than the average of the parents, while sugars, alcohols, and nucleoside metabolites show positive MPH. From the network perspective, metabolites in the photosynthetic pathway show positive MPH, whereas metabolites in the photorespiratory pathway show negative MPH, which corresponds to nonadditive protein abundance and enzyme activities of key enzymes in the respective pathways in the hybrids. Moreover, diurnally expressed proteins that are upregulated in the hybrids are enriched in photosynthesis-related gene-ontology terms. Hybrids may more effectively remove toxic metabolites generated during photorespiration, and thus maintain higher photosynthetic efficiency. These metabolic and proteomic resources provide unique insight into heterosis and its utilization for high yielding maize and other crop plants.

Sialic acids are sometimes 9-O-acetylated in a developmentally regulated and cell-type-specific manner. Cells naturally expressing the disialoganglioside GD3 often O-acetylate the terminal sialic acid residue, giving 9-O-acetyl-GD3 (9AcGD3), a marker of neural differentiation and malignant transformation. We also reported that Chinese hamster ovary cells transfected with GD3 synthase can spontaneously O-acetylate some of the newly synthesized GD3. It is unclear whether such phenomena result from induction of the 9-Oacetylation machinery and whether induction is caused by the GD3 synthase protein or by the GD3 molecule itself. We now show that exogenously added GD3 rapidly incorporates into the plasma membrane of Chinese hamster ovary cells, and 9AcGD3 is detected after ϳ6 h. The incorporated GD3 and newly synthesized 9AcGD3 have a half-life of ϳ24 h. This phenomenon is also seen in other cell types, such as human diploid fibroblasts. Inhibitors of gene transcription, protein translation, or endoplasmic reticulum-to-Golgi transport each prevent induction of 9-O-acetylation, without affecting GD3 incorporation. Inhibition of the initial clathrin-independent internalization of incorporated GD3 also blocks induction of 9-O-acetylation. Thus, new synthesis of one or more components of the 9-O-acetylation machinery is induced by incorporation and internalization of GD3. Prepriming with structurally related gangliosides fails to accelerate the onset of 9-O-acetylation of subsequently added GD3, indicating a requirement for specific recognition of GD3. To our knowledge, this is the first example wherein a newly expressed or exogenously introduced ganglioside induces de novo synthesis of an enzymatic machinery to modify itself, and the first evidence for a mechanism of induction of sialic acid O-acetylation.Gangliosides are glycosphingolipids containing one or more sialic acids and are commonly found on the outer leaflet of the plasma membrane (1, 2). With ceramide tails embedded in the membrane bilayer and glycans protruding from the surface, gangliosides are often clustered in microdomains and lipid rafts. Besides being major structural components in neural cell membranes, gangliosides have been found to play important roles in cell adhesion, cell recognition, signal transduction, and neural development (3-6). Early studies showed that when gangliosides in micellar form are exogenously added to cell culture media, they are capable of first becoming associated with cell surface proteins (a trypsin-sensitive component) and subsequently becoming inserted into the plasma membrane (becoming trypsin-resistant) (7).The latter molecules are then indistinguishable from their endogenous counterparts that are synthesized in the same cell (8,9). This technique has since been widely employed to study the metabolic fate and functions of gangliosides. Radioactive and fluorescently labeled gangliosides have been used to elegantly trace the internalization route of exogenous gangliosides, demonstrating that such gangliosides are intern...

No abstract

9-O-Acetylation is a common sialic acid modification, expressed in a developmentally regulated and tissue/ cell type-specific manner. The relevant 9-O-acetyltransferase(s) have not been isolated or cloned; nor have mechanisms for their regulation been elucidated. We previously showed that transfection of the GD3 synthase (ST8Sia-I) gene into Chinese hamster ovary (CHO)-K1 cells gave expression of not only the disialoganglioside GD3 but also 9-O-acetyl-GD3. We now use differential display PCR between wild type CHO-K1 cells and clones stably expressing GD3 synthase (CHO-GD3 cells) to detect any increased expression of other genes and explore the possible induction of a 9-O-acetyltransferase. The four CHO mRNAs showing major upregulation were homologous to VCAM-1, Tis21, the KCprotein-like protein, and a functionally unknown type II transmembrane protein. A moderate increase in expression of the FxC1 and SPR-1 genes was also seen. Interestingly, these are different from genes observed by others to be up-regulated after transfection of GD3 synthase into a neuroblastoma cell line. We also isolated a CHO-GD3 mutant lacking 9-O-acetyl-GD3 following chemical mutagenesis (CHO-GD3-OAc ؊ ). Analysis of the above differential display PCR-derived genes in these cells showed that expression of Tis21 was selectively reduced. Transfection of a mouse Tis21 cDNA into the CHO-GD3-OAc ؊ mutant cells restored 9-O-acetyl-GD3 expression. Since the only major gangliosides expressed by CHO-GD3 cells are GD3 and 9-O-acetyl-GD3 (in addition to GM3, the predominant ganglioside type in wildtype CHO-K1 cells), we conclude that GD3 enhances its own 9-O-acetylation via induction of Tis21. This is the first known nuclear inducible factor for 9-O-acetylation and also the first proof that 9-O-acetylation can be directly regulated by GD3 synthase. Finally, transfection of CHO-GD3-OAc ؊ mutant cells with ST6Gal-I induced 9-O-acetylation specifically on sialylated N-glycans, in a manner similar to wild-type cells. This indicates separate machineries for 9-O-acetylation on ␣2-8-linked sialic acids of gangliosides and on ␣2-6-linked sialic acids on N-glycans.Sialic acids are a family of 9-carbon carboxylated monosaccharides typically located at the termini of mammalian cell surface sugar chains on both glycoproteins and glycolipids. N-acetylneuraminic acid, the most common sialic acid, is subjected to various modifications in vivo (1-5). One of the most prevalent modifications is O-acetylation of the hydroxyl group at the 9-carbon position. This modification is known to reduce or abolish the recognition of sialic acid residues by sialidases (2, 4, 6, 7), by certain sialic acid-binding lectins like Siglecs (4, 5, 8 -10), and by several viral recognition proteins (4,5,11,12). Conversely, other viruses require 9-O-acetylation for recognition of their target cells (4,5,(13)(14)(15)(16). It is also known that 9-O-acetylation is regulated during development and aberrantly expressed in melanomas and basal cell carcinomas (17)(18)(19)(20). These findings ind...