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FUNGAL STRUCTURE AND ULTRASTRUCTURE 65

molecule of a sister protein ββ-tubulinto form a dimer;
then the dimers polymerize to form tubulin chains. The
microtubules formed by this process interact with two
mechanochemical enzymes (motor proteins), kinesin
and dynein, and probably help to transport organelles
within the fungal hypha.
Actin microfilaments are much narrower than
microtubules, being about 5– 8 nm diameter. In the
slime mould Physarum polycephalumthe actin micro-
filaments are known to function in cytoplasmic
contraction, when actin associates with its motor pro-
tein, myosin. This could also be true of filamentous
fungi, where myosin-like proteins have recently

been detected. However, the most compelling evidence

for a role of the actin cytoskeleton in organizing

fungal growth has come from studies on the yeast,

Saccharomyces cerevisiae. This fungus has microtubules

but, in contrast to mycelial fungi, the microtubules

of yeast do not seem to be involvedin the transport

of major organelles; instead, they seem to function

mainly in orientation of the mitotic spindle. Yet, actin

cables do play a major role in directing secretory vesi-

cles to growth sites in yeast, and by analogy actin might

play a similar role in distributing the apical vesicles to

sites of wall growth in hyphal tips.

In later chapters we return to the roles of the

cytoskeleton in fungal growth (Chapter 4), differ-

entiation (Chapter 5), and the behavior of fungal

zoospores (Chapter 10). But we end this chapter by not-

ing that fungal tubulins differ from those of plants and

animals. They are inhibited by the antibiotic griseofulvin

and by the benzimidazole fungicides, whereas plant and

animal tubulins are insensitive to these compounds. This

is why griseofulvin can be used to treat the dermato-

phytic (ringworm) fungal infections of humans, and

why the benzimidazoles can be used to treat fungal

infections of plants (Chapter 17). Conversely, the

fungal tubulins (with the exception of Oomycota) are

unaffected by colchicine(the toxin from the autumn

crocus) which inhibits nuclear division in plant and

animal cells.

Cited references

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ization of hyphal tips among fungi: vesicles and

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Heath, I.B. (1978) Nuclear Division in the Fungi. Academic

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Heath, I.B. (1994) The cytoskeleton in hyphal growth,

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Fig. 3.20A fungal microtubule (arrowhead) closely asso-
ciated with a membrane-bound cellular organelle termed
a multivesicular body (MVB). Such close association indi-
cates a potential role for moving the organelle, through
the activities of motor proteins. (Courtesy of R. Roberson;
from Roberson & Fuller 1988.)