Growth, Differentiation and Sexuality

56 J.H. Sietsma and J.G.H. Wessels
partly present as crystalline material at the outer
wall surface (Wessels et al. 1972). This type of glucan
usually contains minor amounts of (1-4)-αlinkages,
which have a function in the initiation of
synthesis, similar to that found for bacterial glycogen
synthesis (Grün et al. 2005). Also, part of the
(1-3)-β-glucanisusuallyfoundtobesolubleinalkali
(Sietsma and Wessels 1981).
The alkali-insoluble fraction of the wall
contains glucan and aminosugar polymers. The
glucan consists of (1-3)-β-/(1-6)-β-linked glucose
residues and is apparently highly branched.
The aminosugars in the alkali-insoluble fraction
are generally considered to derive from chitin,
(1-4)-β-linked poly-N-acetyl-glucosamine. However,
it is difficult to ascertain whether these
aminosugars are present as N-acetyl-glucosamine
or glucosamine, because an accurate estimation
of the aminosugar content is usually done after
complete acid hydrolysis, a procedure removing
any acetyl groups. Digestion of samples
with chitinase usually yields chitobiose, indicating
the presence of extensive stretches of
poly-N-acetyl-glucosamine chains. However, this
enzymic treatment usually fails to hydrolyse the
aminosugar-containing polymers completely,
probably because of the close association with
the (1-3)-β-glucanandthepossiblepresenceof
non-acetylated glucosamine residues. Successive
treatments with nitrous acid, which degrades glucosaminoglycans
at non-acetylated glucosamine
residues, and chitinase not only degrades the glucosaminoglycan
but also solubilises all the glucan
(Sietsma and Wessels 1979, 1981, 1990; Kamada
and Takemaru 1983; Suarit et al. 1988; Mol and
Wessels 1987; Mol et al. 1988; van Pelt-Heerschap
and Sietsma 1990), suggesting the occurrence of
covalent linkages between the β-glucan and the
glucosaminoglycan.
In members of the Zygomycetes the hyphal
wall contains, in addition to chitin, long stretches
of (1-4)-β-linked glucosamine residues (chitosan),
which can be isolated from these walls as an
alkali-insoluble but acid-soluble polymer (Kreger
1954; Bartnicki-Garcia and Nickerson 1962). At
least in Mucor mucedo, glucosaminoglycans occur
containing both acetylated and non-acetylated
glucosamine residues (Datema et al. 1977b).
Destruction of these cationic glucosaminoglycans
by nitrous acid released a heteropolymer
containing glucuronic acid, fucose, mannose and
some galactose (Datema et al. 1977a). This acidic
heteropolymer was apparently held insoluble
in the wall by ionic linkage to the polycationic
glucosaminoglycan.
IV. Biosynthesis of Fungal Walls
The tubular form of the hypha must be the consequence
of the way the wall is synthesised and
assembled at the tip. At the base of the extension
zone, the wall must have enough strength to withstand
the turgor pressure.
Not much is known about the role of (1-3)α-glucan,
the prominent alkali-soluble component
of the wall of many fungi. In Schizosaccharomyces
pombe walls, this glucan has an essential structural
role and is not covalently linked to other wall
components (Hochstenbach et al. 1998; Grün et al.
2005). However, this organism is an exception because
chitin, as a structural component, is here
presentatverylowlevels,ifatall(SietsmaandWessels
1990). Mutants of Aspergillus nidulans unable
to synthesise α-glucan have been described (Zonneveld
1974; Polacheck and Rosenberger 1977), and
these were affected in cleistothecia formation but
no effects on osmotic sensitivity or hyphal morphogenesis
were reported. Also, inhibition of synthesis
of this glucan by 2-deoxyglucose during cell
wall regeneration by protoplasts of S. commune
had no effect on the regeneration of hyphae in osmotically
stabilised medium (Sietsma and Wessels
1988). These findings suggest that this glucan does
not play a significant role in hyphal morphogenesis.
On the other hand, when during protoplast
regeneration in S. commune chitin synthesis was
inhibited by polyoxin-D, no hyphal tubes were
formed (Sonnenberg et al. 1982). Under these
conditions, the protoplasts became osmotically
stable but the wall contained (1-3)-α-glucan only.
The β-glucan was normally synthesised but as
a water/alkali-soluble component, whereas no
alkali-insoluble glucan was formed, apparently
because the aforementioned linkage to chitin
could not be established. Similar observations
have been made during regeneration of the wall
in Candida albicans protoplasts (Elorza et al.
1987). Pulse-chase experiments with regenerating
protoplasts (Sonnenberg et al. 1982) and growing
hyphae (Wessels et al. 1983) of S. commune have
shown that the water- and alkali-soluble (1-3)-βglucan
is indeed a precursor of the alkali-insoluble
β-glucan.