The Golgi apparatus maintains a highly organized structure in spite of the intense membrane traffic which flows into and out of this organelle. Resident Golgi proteins must have localization signals to ensure that they are targeted to the correct Golgi compartment and not swept further along the secretory pathway. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recycling trans-Golgi network proteins, peripheral membrane proteins, receptors and viral glycoproteins. Recent studies indicate that there are a number of different Golgi localization signals and mechanisms for retaining proteins to the Golgi apparatus. This review focuses on the current knowledge in this field.
Extracellular, high-molecular-weight glycoconjugates were extracted from cleaned frustules of Craspedostauros australis with 8 M urea in 50% formic acid (UFE) or with 2 M guanidine-HCl (GU). The weight ratio of protein : carbohydrate : sulphate : phosphate was 42 : 8 : 3 : 1 in the GU extract. The three dominant molecules in the extracts (one of 112 kDa and two 4 220 kDa) showed slightly increased mobility by SDS-polyacrylamide gel electrophoresis following deglycosylation with anhydrous HF, demonstrating that they were glycoproteins. The carbohydrates were studied by a combination of carboxylreduction of uronic acids, N-acetylation of aminosugars, and characterization of the constituent sugars as alditol acetates or as trimethyl silyl derivatives of their corresponding methyl glycosides. Sixteen sugars were identified in the extracts, including two pentoses, four hexoses, two 6-deoxyhexoses, five mono-O-methylated sugars, two uronic acids, and one amino sugar. The predominant sugar was xylose (37-41 mol%), with significant proportions of galactose (13-15 mol%), rhamnose (10-15 mol%), and mannose (10 mol%) also present. The linkage and substitution patterns of the sugars characterized by methylation analysis were complex. The predominant residue was terminal xylopyranose (16 mol%) followed by 2-and 4linked xylopyranose (8 and 7 mol%, respectively). Terminal residues outnumbered the available branch points by ca. 4 : 1, indicating that the carbohydrates were mainly short-chain oligosaccharides. Together with previous studies of immunolocalization and analysis of physical properties, the composition of the C. australis glycoproteins suggests that they serve mucin-like functions on the cell surface but also supports the proposal that glycoproteins are involved in diatom cell adhesion.
The Lec1 Chinese hamster ovary (CHO) mutant is a leuco-phytohemagglutinin resistant cell line unable to synthesize complex and hybrid N-glycans due to the lack of N-acetylglucosaminyltransferase I (GnTI) activity. Here we have identified the lec1 mutation. ) are correctly localized to the Golgi apparatus, indicating that the inactive GnTI molecules are sufficiently well folded for efficient transport from the endoplasmic reticulum. These results demonstrate that the lec1 mutation is a point mutation and that Cys 123 is a critical residue for GnTI activity.
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