Flocculation of Saccharomyces cerevisiae is induced by transformation with the GAP1 gene from Kluyveromyces marxianus

The Crystal and Solution Structures of Glyceraldehyde-3-phosphate Dehydrogenase Reveal Different Quaternary Structures

The presence of an isoform of glyceraldehyde-3-phosphate dehydrogenase (kmGAPDH1p) associated with the cell wall of a flocculent strain of Kluyveromyces marxianus was the first report of a non-cytosolic localization of a glycolytic enzyme, but the mechanism by which the protein is transported to the cell surface is not known. To identify structural features that could account for the multiple localizations of the protein, the three-dimensional structure of kmGAPDH1p was determined by x-ray crystallography and small angle x-ray scattering. The x-ray crystallographic structure of kmGAPDH1p revealed a dimer, although all GAPDH homologs studied thus far have a tetrameric structure with 222 symmetry. Interestingly, the structure of kmGAPDH1p in solution revealed a tetramer with a 70 degrees tilt angle between the dimers. Moreover, the separation between the centers of the dimers composing the kmGAPDH1p tetramer diminished from 34 to 30 A upon NAD(+) binding, this latter value being similar to the observed in the crystallographic models of GAPDH homologs. The less compact structure of apo-kmGAPDH1p could already be the first image of the transition intermediate between the tetramer observed in solution and the dimeric form found in the crystal structure, which we postulate to exist in vivo because of the protein's multiple subcellular localizations in this yeast species.

Pathogen elicitor-induced changes in the maize extracellular matrix proteome

The extracellular matrix is a vital compartment in plants with a prominent role in defence against pathogen attack. Using a maize cell suspension culture system and pathogen elicitors, responses to pathogen attack that are localised to the extracellular matrix were examined by a proteomic approach. Elicitor treatment of cell cultures induced a rapid change in the phosphorylation status of extracellular peroxidases, the apparent disappearance of a putative extracellular beta-N-acetylglucosamonidase, and accumulation of a secreted putative xylanase inhibitor protein. Onset of the defence response was attended by an accumulation of glyceraldehyde-3-phosphate dehydrogenase and a fragment of a putative heat shock protein. Several distinct spots of both proteins, which preferentially accumulated in cell wall protein fractions, were identified. These three novel observations, viz. (i) secretion of a new class of putative enzyme inhibitor, (ii) the apparent recruitment of classical cytosolic proteins into the cell wall and (ii) the change in phosphorylation status of extracellular matrix proteins, suggest that the extracellular matrix plays a complex role in defence. We discuss the role of the extracellular matrix in signal modulation during pathogen-induced defence responses.

Starvation and temperature upshift cause an increase in the enzymatically active cell wall-associated glyceraldehyde-3-phosphate dehydrogenase protein in yeast

The cell wall-associated glyceraldehyde-3-phosphate dehydrogenase (cwGAPDH) activity in Saccharomyces cerevisiae increases (two- to 10-fold, depending on the strain) in response to starvation and temperature upshift. Assays using transformants carrying pTDH, a yeast centromer derivative plasmid containing the Candida albicans TDH3 gene (encoding GAPDH) fused in frame with the yeast SUC2-coding region for internal invertase, showed that starvation and/or temperature upshift result in a similar increase in both cwGAPDH and cell wall-associated invertase activities. In addition, this incorporation of GAPDH protein into the cell wall in response to stress does not require (i) de novo protein synthesis, indicating that preexisting cytosolic enzyme is incorporated into the cell wall, (ii) nor the participation of the ubiquitin yeast stress response system, as no differences were observed between wild-type and polyubiquitin-depleted (Deltaubi4) strains.

Candida albicans TDH3 gene promotes secretion of internal invertase when expressed in Saccharomyces cerevisiae as a glyceraldehyde-3-phosphate dehydrogenase-invertase fusion protein

We have checked the ability of the Candida albicans GAPDH polypeptide, which lacks a conventional N-terminal signal peptide, to reach the cell wall in Saccharomyces cerevisiae by using an intracellular form of the yeast invertase as a reporter protein. A hybrid TDH3-SUC2 gene containing the C. albicans TDH3 promoter sequences and a coding region encoding a fusion protein formed by the C. albicans GAPDH polypeptide, fused at its C-terminus with the yeast internal invertase, was constructed in a centromer derivative plasmid and transformed into a Suc(-) S. cerevisiae strain. Transformants displayed invertase activity measured in intact whole cells, and were able to grow on sucrose as the sole fermentable carbon source. Northern blot analysis with both TDH3 and SUC2 probes detected a single mRNA species of the expected size (about 2.7 kb), and Western immunoblot analysis of cell-free extracts, using a monoclonal antibody (mAb49) against a C. albicans GAPDH epitope, showed the presence of a 90 kDa polypeptide corresponding to the GAPDH-invertase fusion protein. This indicates that the TDH3 gene is able to direct part of the encoded gene product to the cell wall, and that any putative motifs for this targeting should be within the GAPDH amino acid sequence. Further analysis, using the same approach, of a panel of seven N- and C-terminal GAPDH truncates revealed that the region required for the cell wall targeting is located within the N-terminal half of the protein.

The glyceraldehyde-3-phosphate dehydrogenase polypeptides encoded by the Saccharomyces cerevisiae TDH1, TDH2 and TDH3 genes are also cell wall proteins

The authors show that the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) of Saccharomyces cerevisiae, previously thought to be restricted to the cell interior, is also present in the cell wall. GAPDH activity, proportional to cell number and time of incubation, was detected in intact wild-type yeast cells. Intact cells of yeast strains containing insertion mutations in each of the three structural TDH genes (tdh1, tdh2 and tdh3) and double mutants (tdh1 tdh2 and tdh1 tdh3) also displayed a cell-wall-associated GAPDH activity, in the range of parental wild-type cells, although with significant differences among strains. A cell wall location of GAPDH was further confirmed in wild-type and tdh mutants by indirect immunofluorescence and flow cytometry analysis with a polyclonal antibody against S. cerevisiae GAPDH. By immunoelectron microscopy, the GAPDH protein was detected at the outer surface of the cell wall of wild-type cells, as well as in the cytoplasm. Western immunoblot analysis of cell wall extracts and cytosol showed that Tdh2 and Tdh3 polypeptides are present in the cell wall, as well as in the cytosol, of exponentially growing cells. Tdh1 is only detected in stationary-phase cells, again in both cytosol and cell wall extracts. The results incorporate the GAPDH of S. cerevisiae, encoded by TDH1-3, into the newly emerging family of multifunctional cell-wall-associated GAPDHs which retain their catalytic activity.

A proteomic approach for the study of Saccharomyces cerevisiae cell wall biogenesis

In fungi, cell shape is determined by the presence of a rigid cell wall which separates the cell from the extracellular medium. This highly dynamic structure is essential for the maintenance of cell integrity and is involved in several phenomena such as flocculation, adherence and pathogenicity. The composition of the fungal cell wall is well known, but issues such as the assembly and remodeling of its components remain poorly understood. In an attempt to study the de novo construction of the yeast cell wall, we have undertaken a large-scale proteomic approach to analyze the proteins secreted by regenerating protoplasts. Upon incubation of protoplasts in regenerating conditions, numerous proteins are secreted into the culture medium. These presumably include proteins destined for the cell wall, comprising both structural proteins as well as enzymes involved in cell wall biogenesis. This work reports the establishment of a reference map of proteins secreted by regenerating protoplasts by means of two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) and their identification by mass spectrometry. Thirty-two different proteins have been identified, including known cell wall proteins, glycolytic enzymes, heat shock proteins, and proteins involved in several other processes. Using this approach, novel proteins possibly involved in cell wall construction have also been identified. This reference map will allow comparative analyses to be carried out on a selected collection of mutants affected in the cell wall.