GPI-Modified Proteins Non-covalently Attached to Saccharomyces cerevisiae Yeast Cell Wall.

Yeast cell wall GPI-anchored proteins lack the lipid part of the anchor and are covalently bound to the high-molecular-weight polysaccharides glucan and/or chitin through the mannose residues. They perform many functions, including participation in the cell wall molecular ensemble formation and providing cell resistance to stress. In this work, we identified a pool of GPI-modified proteins firmly bound to the cell wall by non-covalent interactions with the high-molecular-weight structural polysaccharides. We believe that the detected proteins are intermediate forms in the processing of the cell wall GPI-proteins, since they had already lost the lipid part of the GPI anchor and are absent in the lipoprotein fraction extracted according to Folch, but were not yet incorporated into the cell wall by the covalent binding to high-molecular-weight polysaccharides because they could be extracted into water by heating of delipidized cell walls. This group of previously unknown proteins might be present in the cell wall in a form of lipid-associated microcompartments represented by transport vesicles recently found in yeast. GPI-modified proteins non-covalently attached to the high-molecular-weight polysaccharides were found in the cell walls of both the parent strain and yeast devoid of glucanosyltransglycosylase Bgl2, which indicates that the pathway of their incorporation into the cell wall is independent on this enzyme.

[Molecular Organization of Yeast Cell Envelope].

This review summarizes the main achievements of recent years in molecular organization research of yeast cell surface, i.e., the compartment that consists of the coordinately functioning plasma membrane, periplasmic space, and cell wall. There are data on vesicular transport to the external environment through the cell wall and the formation of channels in the wall, which indicate the possibility of dynamic rearrangements of the molecular structure of the yeast cell wall. There is an idea about the mosaic arrangement of the compartments of the plasma membrane. The hypothesis has been suggested on the heterogeneity of the molecular structure of the cell wall, which is usually considered as uniform except for the budding zones. The groups of proteins that form the molecular assembly of the yeast cell surface have been described. Special attention has been paid for proteins with amyloid properties, including Bgl2p glucanosyltransglycosylase, which is important for virulence in pathogenic yeast, and Gas1p, the first of the studied proteins of the cell surface, which is involved in the regulation of ribosomal DNA transcriptional silencing. The data on the structure of receptors localized on the cell surface and the "moonlight" proteins, involved in the cell stress response of yeasts, have been given.

C-Terminal sequence is involved in the incorporation of Bgl2p glucanosyltransglycosylase in the cell wall of Saccharomyces cerevisiae.

A cell wall (CW) provides a protective barrier for a yeast cell and is a firm structure that nevertheless dynamically changes during cell's growth. Bgl2p is a non-covalently anchored glucanosyltransglycosylase in the CW of the yeast Saccharomyces cerevisiae. The mode of its anchorage is poorly understood, while its association with CW components is tight and resistant to 1-h treatment with 1% SDS at 37°C. In order to demarcate the potential structural block responsible for incorporation of Bgl2p into the CW, bioinformatics analysis of its sequence was performed, and a conservative structural region was identified in the C-terminal region of Bgl2p, which was absent in its homologues in S. cerevisiae, the Scw4p and Scw10p. Deletion of this region disrupted the incorporation of Bgl2p into the CW and led to release of this protein through the CW into the culture medium. Two left-handed polyproline-II helices were identified in the C-terminal region of the structure model of a wild-type Bgl2p. These helices potentially formed binding sites, which were absent in the truncated protein. Using immune fluorescence microscopy, we demonstrated that C-truncated Bgl2p was exported into culture medium and lost its ability to form fibrils described earlier. It was also shown that the C-terminal truncation of Bgl2p led to a more severe decrease of cell survivability in extreme conditions than BGL2 deletion.