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y Structure of repeating mannan outer chain (x = 10 -15)

Fig. 4.33 The molecular structure of a cell wall mannoprotein.

species is due to the polymorphism of yeast mannoproteins. Numerous mannoprotein mutants (mnn) have been isolated and characterised. Amongst other studies, these have allowed the unravelling of minimum receptor structure that will support floc-culation (Section 4.4.6.2).

4.4.2.3 Chit in. As well as the yeast cell wall, chitin is found in the exoskeleton of crustaceans and insects. However, it is quite distinct from the other polysaccharides found in the cell wall. Structurally it is comparatively simple, being a linear polymer of p-(l ->4) linked N-acetylglucosamine. Although chitin is believed to play no role in the structure of the cell wall, more recent evidence with chitin mutants has suggested it may be important in wall structure (Stratford, 1994). Chitin is found, almost exclusively (c. 90% of total), in scars left on the mother cell surface after the cell has undergone reproduction through budding (see Section 4.3.3.1). These 'bud scars', which resemble volcanic craters, are left every time a cell buds. The chitin-rich scar is all that is left of the primary septum between mother and daughter cell (Cabib et al., 1982; Fleet, 1991).

Robinow and Johnson (1991) noted somewhat tartly that 'considering how truly unimportant yeast scars are, it is remarkable that they have a literature all their own'. With this admonishment in mind, a few comments about bud scars are appropriate! First, the view that the cell's surface limits the number of bud scars is not the case (Robinow & Johnson, 1991). Second, the number of bud scars provides a useful measure of cell age. Although a seemingly trite comment, S. cerevisiae is a model organism to study cellular senescence (see Section 4.3.3.4) which has triggered some valuable insights into yeast cell age during brewery handling (Barker & Smart, 1996; Deans et al., 1997).

The use of microscopy (conventional light or, occasionally electron microscopy) (Fig. 4.34) to visualise bud scars is limited by the '2D nature of the image'. Work at

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