BBA - General Subjects (v.1673, #1-2)
Editorial Board (ii).
Publisher's Announcement (vi).
Editorial by Gerald W Hart (1).
Intracellular hyaluronan: a new frontier for inflammation? by Vincent C Hascall; Alana K Majors; Carol A de la Motte; Stephen P Evanko; Aimin Wang; Judith A Drazba; Scott A Strong; Thomas N Wight (3-12).
A variety of obstacles have hindered the ultrastructural localization of hyaluronan (HA). These include a lack of adequate fixation techniques to prevent the loss of HA, the lack of highly sensitive and specific probes, and a lack of accessibility due to the masking of HA by HA-binding macromolecules such as proteoglycans and glycoproteins. Despite these problems, a number of studies, both biochemical and histochemical, have been published indicating that HA is not restricted to the extracellular milieu, but is also present intracellularly. This review focuses on the possible functions of intracellular HA, its potential relationships to extracellular HA structures, and implications for inflammatory processes.
Keywords: Hyaluronan; Proteoglycan; Glycoprotein;
O-GlcNAc a sensor of cellular state: the role of nucleocytoplasmic glycosylation in modulating cellular function in response to nutrition and stress by Natasha E Zachara; Gerald W Hart (13-28).
Myriad nuclear and cytoplasmic proteins in metazoans are modified on Ser and Thr residues by the monosaccharide O-linked β-N-acetylglucosamine (O-GlcNAc). The rapid and dynamic change in O-GlcNAc levels in response to extracellular stimuli, morphogens, the cell cycle and development suggests a key role for O-GlcNAc in signal transduction pathways. Modulation of O-GlcNAc levels has profound effects on the functioning of cells, in part mediated through a complex interplay between O-GlcNAc and O-phosphate. In many well-studied proteins, the O-GlcNAc modification and phosphorylation are reciprocal. That is, they occur on different subsets of the protein population, as the site of attachment occurs on the same or adjacent Ser/Thr residues. Recently, O-GlcNAc has been implicated in the etiology of type II diabetes, the regulation of stress response pathways, and in the regulation of the proteasome.
Keywords: Heat shock; Hyperglycemia; Stress; Post-translational modification; N-acetylglucosamine; O-GlcNAc; Phosphorylation;
Cytoplasmic glycosylation of protein-hydroxyproline and its relationship to other glycosylation pathways by Christopher M West; Hanke van der Wel; Slim Sassi; Eric A Gaucher (29-44).
The Skp1 protein, best known as a subunit of E3SCF-ubiquitin ligases, is subject to complex glycosylation in the cytoplasm of the cellular slime mold Dictyostelium. Pro143 of this protein is sequentially modified by a prolyl hydroxylase and five soluble glycosyltransferases (GT), to yield the structure Galα1,Galα1,3Fucα1,2Galβ1,3GlcNAcα1-HyPro143. These enzymes are unusual in that they are expressed in the cytoplasmic compartment of the cell, rather than the secretory pathway where complex glycosylation of proteins usually occurs. The first enzyme in the pathway appears to be related to the soluble animal prolyl 4-hydroxylases (P4H), which modify the transcriptional factor subunit HIF-1α in the cytoplasm, and more distantly to the P4Hs that modify collagen and other proteins in the rER, based on biochemical and informatics analyses. The soluble αGlcNAc-transferase acting on Skp1 has been cloned and is distantly related to the mucin-type polypeptide N-acetyl-α-galactosaminyltransferase in the Golgi of animals. Its characterization has led to the discovery of a family of related polypeptide N-acetyl-α-glucosaminyltransferases in the Golgi of selected lower eukaryotes. The Skp1 GlcNAc is extended by a bifunctional diglycosyltransferase that sequentially and apparently processively adds β1,3Gal and α1,2Fuc. Though this structure is also formed in the animal secretory pathway, the GTs involved are dissimilar. Conceptual translation of available genomes suggests the existence of this kind of complex cytoplasmic glycosylation in other eukaryotic microorganisms, including diatoms, oomycetes, and possibly Chlamydomonas and Toxoplasma, and an evolutionary precursor of this pathway may also occur in prokaryotes.
Keywords: Glycosyltransferase evolution; Multifunctional protein; Mucin-type O-glycosylation; Thalassiosira; Phytophthora sojae; Yersinia;
Glycogenin: the primer for mammalian and yeast glycogen synthesis by Joseph Lomako; Wieslawa M Lomako; William J Whelan (45-55).
Glycogen synthesis, whether in mammalian tissue, yeast, or Agrobacterium tumefaciens or other bacteria, is initiated by autoglucosylation of a protein. Initiation in muscle, by a self-glucosylating protein, glycogenin-1, is the most thoroughly studied system, as is described here. These relatively recent findings have prompted a rekindling of interest in the intermediates lying between the primer and mature mammalian glycogen.
Keywords: Self-glucosylation; Autoglucosylation; Glycogenin-1; Glycogenin-2; Proglycogen; Macroglycogen; Glycogen synthase primer;
CMP-sialic acid synthetase of the nucleus by Edward L Kean; Anja K Münster-Kühnel; Rita Gerardy-Schahn (56-65).
Sialic acids of cell surface glycoconjugates play a pivotal role in the structure and function of animal cells and in some bacterial pathogens. The pattern of cell surface sialylation is species specific, and, in the animal, highly regulated during embryonic development. A prerequisite for the synthesis of sialylated glycoconjugates is the availability of the activated sugar-nucleotide cytidine 5′-monophosphate N-acetylneuraminic acid (CMP-NeuAc), which provides the substrate for sialyltransferases. Trials to purify the enzymatic activity responsible for the synthesis of CMP-NeuAc from different animal sources demonstrated that the major localisation of the enzyme is the cell nucleus. These earlier findings were confirmed when the murine CMP-NeuAc synthetase was cloned and the subcellular transport of recombinant epitope tagged forms visualised by indirect immunofluorescence. Today, the primary sequence elements that direct murine CMP-NeuAc synthetase into the cell nucleus are known, however, information regarding the physiological relevance of the nuclear destination is still not available. With this article, we provide a detailed review on earlier and recent findings that identified and confirmed the unusual subcellular localisation of the CMP-NeuAc synthetase. In addition, we take the advantage to discuss most recent developments towards understanding structure–function relations of this enzyme.
Keywords: CMP-sialic acid; CMP-sialic acid synthetase; Pyrophosphorylase; Nuclear transport; Nuclear localisation signal;
Large clostridial cytotoxins: cellular biology of Rho/Ras-glucosylating toxins by Jörg Schirmer; Klaus Aktories (66-74).
Mono-O-glycosylation of eukaryotic target proteins is the molecular mechanism of bacterial protein toxins of the family of large clostridial cytotoxins. This toxin family encompasses several high molecular mass proteins (>250 kDa) of various Clostridia species that are implicated in severe human diseases. Toxin A and toxin B from Clostridium difficile are the causative agents of pseudomembranous colitis and antibiotic-associated diarrhea. Lethal toxin and hemorrhagic toxin from Clostridium sordellii as well as α-toxin from Clostridium novyi are involved in the gas gangrene syndrome. The common mode of action of large clostridial cytotoxins is elicited by specific glycosylation of small GTP-binding proteins in the cytosol of target cells using activated nucleotide sugars as cosubstrates. Specific modification at a single threonine residue in the small GTPases renders these important key players of various signaling pathways inactive. This minireview intends to give an overview on structure–function analysis and mode of action of the large clostridial cytotoxins, as well as on the resulting functional consequences of glycosylation of target proteins.
Keywords: Clostridium; Clostridium difficile toxin; Cytotoxin; Glycosylation; Rho protein; Small GTPase; UDP glycosyltransferase;
Nucleocytoplasmic lectins by John L Wang; Richard M Gray; Kevin C Haudek; Ronald J Patterson (75-93).
This review summarizes studies on lectins that have been documented to be in the cytoplasm and nucleus of cells. Of these intracellular lectins, the most extensively studied are members of the galectin family. Galectin-1 and galectin-3 have been identified as pre-mRNA splicing factors in the nucleus, in conjunction with their interacting ligand, Gemin4. Galectin-3, -7, and -12 regulate growth, cell cycle progression, and apoptosis. Bcl-2 and synexin have been identified as interacting ligands of galectin-3, involved in its anti-apoptotic activity in the cytoplasm. Although the annexins have been studied mostly as calcium-dependent phospholipid-binding proteins mediating membrane-membrane and membrane-cytoskeleton interactions, annexins A4, A5 and A6 also bind to carbohydrate structures. Like the galectins, certain members of the annexin family can be found both inside and outside cells. In particular, annexins A1, A2, A4, A5, and A11 can be found in the nucleus. This localization is consistent with the findings that annexin A1 possesses unwinding and annealing activities of a helicase and that annexin A2 is associated with a primer recognition complex that enhances the activity of DNA polymerase α. Despite these efforts and accomplishments, however, there is little evidence or information on an endogenous carbohydrate ligand for these lectins that show nuclear and/or cytoplasmic localization. Thus, the significance of the carbohydrate-binding activity of any particular intracellular lectin remains as a challenge for future investigations.
Keywords: Carbohydrate-binding protein; Galectin; Carbohydrate recognition;
Glyco-dependent nuclear import of glycoproteins, glycoplexes and glycosylated plasmids by Michel Monsigny; Christine Rondanino; Eric Duverger; Isabelle Fajac; Annie-Claude Roche (94-103).
This short review deals with some properties of nuclear sugar-binding proteins also called nuclear lectins, the sugar-dependent nuclear import of neoglycoproteins and the attempts of using this pathway to enhance the nuclear import of plasmids in order to hopefully increase the expression of transferred genes.
Keywords: Glycofection; Lectin; Neoglycoprotein; NLS; Nuclear import; Plasmid;