untitled

FUNGAL GROWTH 71

around a chitosome is not a phospholipid membrane,
so chitosomes might be packaged within phospho-
lipid membranes for transport to the tip – perhaps in
the multivesicular bodies that are sometimes seen in
electron micrographs (see Fig. 3.20).
The zymogen form of chitin synthase, when inserted
into the membrane, must be activated by a protease
which probably arrives at the apex in other vesicles.
Then the substrate is delivered from the cytosol to the
inner face of the chitin synthase enzyme (anchored in
the membrane), and chitin chains are synthesized and
extruded from the membrane outer face (Fig. 4.5). The
substrate for chitin synthesis is N-acetylglucosamine,
but it is supplied as a sugar nucleotide, UDP-
N-acetylglucosamine (where UDP = uridine diphos-
phate), with a high-energy bond required for chitin
synthesis as explained in Chapter 6. Clearly, there must
be mechanisms for regulating the activity of chitin
synthase during wall growth. This could be achieved
in a number of ways, including enzyme inhibitors,
because the cytosol is known to contain a chitin syn-
thase inhibitor, which might prevent any “spill-over.”

Glucan synthase

Glucan synthase is the other major enzyme involved
in wall growth. It catalyses the synthesis of β-1,3-
glucan chains, which often comprise the bulk of the
fungal wall. Like chitin synthase, glucan synthase is
thought to arrive in vesicles and becomes inserted in
the plasma membrane at the apex. The substrate for
this enzyme is a sugar nucleotide (UDP-glucose), sup-
plied from the cytosol. However, the activity of glucan
synthase is regulated in a different way from chitin
synthase. The enzyme is composed of two subunits, one
of which (on the membrane outer face) contains the
catalytic site, and the other is a guanosine triphosphate
(GTP) binding protein. So the enzyme is thought to be
activated when GTP arrives at the cytoplasmic face,
then glucan chains are synthesized and extruded into
the wall. The glucan chains seem to undergo further
modification within the wall. In particular, short β-1,6-
linked side chains develop and link the β-1,3-glucan
chains. The number of these branched linkages
increases progressively as the wall matures behind the
apex.

Mannoproteins

Mannoproteins and other glycoproteins form a relatively
small proportion of the total wall composition of fun-
gal hyphae, but are more common in yeasts and in the
yeast-like phase of dimorphic fungi. These glycosy-
lated proteins are among the few wall components that
are pre-formed in the endoplasmic reticulum–Golgi
complex and are delivered to the apex in vesicles.

Cross-linking and maturation of the hyphal wall

Various types of cross-linkage occur between the

major wall polymers after these have been inserted in

the wall, and this seems to occur progressively back from

the hyphal tip. For example, essentially pure glucans

can be extracted from newly formed fungal walls

by using hot alkali, but in the older wall regions an

increasing proportion of the glucan is alkali-insoluble,

apparently because it is complexed with chitin. In

support of this view, the glucans can be extracted after

treating walls with chitinase to degrade the chitin. The

chitin and glucans are linked by covalent bonds. Little

is known about the process except that amino acids may

be involved,because the amino acid lysine is associ-

ated with up to half of the glucan–chitin linkages in

walls of Schizophyllum commune(Basidiomycota). In

addition to these intermolecular bonds, the chitin

chains associate with one another by hydrogen bond-

ing, to form microfibrils. The glucans also associate with

one another. These additional bondings behind the

growing apex could serve to convert the initially

plastic wall into a progressively more cross-linked and

rigidified structure.

Wall lytic enzymes

There are opposing views on whether wall lytic

enzymes are necessary for apical growth. On the one

hand, it has been suggested that the existing wall

must be softened in order for new wall components to

be inserted, in which case wall growth would involve

a balance of wall lysis and wall synthesis. Consistent

with this, chitinase, cellulase(in Oomycota), and

ββ-1,3-glucanaseactivities can be found in hyphal wall

fractions, although these enzymes might exist usually

in a latent form. On the other hand, it has been argued

that the substantial cytoskeleton of tubulins and actin

could help to reinforce the hyphal tip, precluding

the need for a rigid wall and therefore precluding the

need for wall-degrading enzymes. However, there is no

doubt that wall-lytic enzymes would be required for the

production of new tips (new hyphal branches) that

emerge from the previously rigid wall further back

from the hyphal apex.

A steady-state model of wall growth

Wessels (1990) proposed a steady-state model of fun-

gal tip growth that could make the role of tip-located

wall-lytic enzymes unnecessary, and also could ex-

plain several other features of tip growth. According to

this model, the newly formed wall at the extreme tip

is suggested to be viscoelastic, so that the wall poly-

mers flow outwards and backwards as new components