Structure of the Fungal Cell

STRUCTURE OF THE FUNGAL CELL

The typical yeast cell is oval in shape and is surrounded by a rigid cell wall which contains a number of structural polysaccharides and may account for up to 25% of the dry weight of the cell wall (see Figure 4.2). Glucan accounts for 50–60%, mannan for 15–23% and chitin for 1–9% of the dry weight of the wall, respectively, with protein and lipids also present in smaller amounts. The thickness of the cell wall may vary during the life of the cell but the average thickness in the yeast C. albicans varies from 100 to 300 nm. Glucan, the main structural component of the fungal cell wall, is a branched polymer of glucose which exists in three forms in the cell: β-1,6-glucan, β-1,3-glucan and β1,3,-β-1,6-complexed with chitin. Mannan is a polymer of the sugar mannose and is found in the outer layers of the cell wall. The third principal structural component, chitin, is concentrated in bud scars that are areas of the cell from which a bud has detached. Proteins and lipids are also present in the cell wall and under some conditions may represent up to 30% of the cell wall contents. Mannoproteins form a fibrillar layer that radiates from an internal skeletal layer that is formed by the polysaccharide component of the cell wall. The innermost layer is rich in glucan and chitin which provides rigidity to the wall and is important in regulating cell division.

Enzymatic or mechanical removal of the cell wall leaves an osmotically fragile protoplast which will burst if not maintained in an osmotically stabilized environment. Incubation of protoplasts in an osmotically stabilized agar growth medium will allow the re-synthesis of the wall and the resumption of normal cellular functions. The ability to generate fungal protoplasts opens the possibility of fusing these under defined conditions to generate strains with novel biotechnological applications.

The periplasmic space is a thin region that lies directly below the cell wall. It contains secreted proteins that do not penetrate the cell wall and is the location for a number of enzymes required for processing nutrients prior to entry into the cell. The cell membrane or plasmalemma is located directly below the periplasmic space and is a phopholipid bilayer which contains phospholipids, lipids, protein and sterols. The plasmalemma is approximately 10 nm thick and in addition to being composed of phospholipids also contains globular proteins. The dominant sterol in fungal cell membranes is ergosterol which is the target of the antifungal agent amphotericin B. Sterols are important components of the plasmalemma and represent regions of rigidity in the fluidity provided by the phospholipid bilayer.

Most of the cell’s genome is concentrated in the nucleus which is surrounded by a nuclear membrane which contains pores to allow communication with the rest of the cell (see Figure 4.2 ). The nucleus is a discrete organelle and, in addition to being the repository of the DNA, also contains proteins in the form of histones. Yeast chromosomes vary in size from 0.2 to 6 Mb and the number per yeast is also variable with S. cerevisiae having as many as 16 while the fission yeast Sch. pombe has as few as 3. In addition to the genetic material in the nucleus the yeast cell often has extrachromosomal information in the form of plasmids. For example, the 2 μm plasmid is present in S. cerevisiae, although its function is unclear, and there are killer plasmids in the yeast Kluyveromyces lactis which encode a toxin.

Actively respiring fungal cells possess a distinct mitochondrion which has been described as the ‘powerhouse’ of the cell (Figure 4.2). The enzymes of the tricarboxylic acid cycle (Krebs’ cycle) are located in the matrix of the mitochondrion while electron transport and oxidative phosphorylation occur in the mitochondrial inner membrane. The outer membrane contains enzymes involved in lipid biosynthesis. The mitochondrion is a semi-independent organelle as it possesses its own DNA and is capable of producing its own proteins on its own ribosomes which are referred to as mito-ribosomes.

The fungal cell contains a vast number of ribosomes which are usually present in the form of polysomes— lines of ribosomes strung together by a strand of mRNA. Ribosomes are the site of protein biosynthesis. The system which mediates the export of proteins from the cell involves a number of membranous compartments including the Golgi apparatus, the endoplasmic reticulum and the plasmalemma. In addition, the vacuole is employed as a ‘storage space’ where nutrients, hydrolytic enzymes or metabolic intermediates are retained until required.