Another brick in the wall? Recent developments concerning the yeast cell envelope

To a yeast, the cell wall is an important living organelle performing a number of vital functions, including osmotic and physical protection, selective permeability barrier, immobilized enzyme support and cell-cell recognition and adhesion. Our basic model of wall structure involves attachment of secreted mannoproteins to a fibrillar inner layer of beta-glucan. Recent work has emphasised the importance of chitin in lateral walls, examined the mechanisms of attachment of mannoproteins to the various cell wall glucan fractions and elucidated the pathway of beta-glucan synthesis, by means of resistance to glucan-binding killer toxins. The conventional view of wall structure has been challenged by the discovery of a class of GPI-anchored, serine/threonine-rich wall-proteins. It has been suggested, that these proteins are anchored in the plasma membrane, spanning the wall with extended O-glycosylated structures and protruding out into the medium. Examination of these proteins shows a diversity of structures, sizes and behaviour that makes it improbable that these represent a new class of wall proteins. The possible roles of one of these proteins associated with flocculation, Flo1p, are discussed.