Growth, Differentiation and Sexuality

in membrane enlargement and exocytosis would
occur.
The apical vesicles are thought to arise in the
endoplasmic reticulum far behind the tip, pass
throughGolgiequivalents,andbethentransported
vectorially to the apex where they fuse with the
plasma membrane (see The Mycota, Vol. I, 1st edn.,
Chap. 3). In their membranes, these vesicles may
carry proteins destined for the plasma membrane,
such as polysaccharide synthases, ion channels and
ATP-ase. In their lumina, they may carry some wall
components, wall-modifying enzymes, as well as
lytic enzymes destined for excretion to digest polymeric
substrates. The cytological and enzymological
evidence attesting to this view has been reviewed
by Gooday and Gow (1990). As mentioned
above, evidence is accumulating that chitin synthesis
in yeast and at the hyphal tip is regulated by exoand
endocytosis of chitin synthase-containing particles
(see The Mycota, Vol. III, 2nd edn., Chap. 14).
B. TheHyphalWallasaBarrierforPassage
of Proteins
Thecellwallisthelastbarrierproteinssecretedinto
the medium have to take. Slime mutants of N. crassa
whicharedefectiveinwallsynthesisover-excrete
enzymes, including invertase (Bigger et al. 1972;
Casanova et al. 1987; Pietro et al. 1989), alkaline
phosphatase (Burton and Metzenberg 1974), alkaline
protease and aryl-β-glucosidase (Pietro et al.
1989). This suggests that the wall acts as a barrier
in protein excretion. Moreover, the problem of
passage through the wall is compounded by the fact
that excreted proteins are sometimes much larger
than the estimated average pore size of the walls of
yeasts (Scherrer et al. 1974; Cope 1980) and mycelial
fungi (Trevithnick and Metzenberg 1966). These
pore sizes were measured with isolated cell wall
fractions, or determined by the solute exclusion
method, using living hyphae (Money 1990).
On the basis of the existence of a correlation
between the sizes of excreted proteins and numbers
of hyphal tips present, Chang and Trevithick
(1974) have proposed that the proteins are excreted
at the hyphal apices and that the nascent wall at the
apex contains larger pores than the mature subapical
wall. Because of the small proportion of apical
wall material in wall preparations used for poresize
measurements, such large pores would go undetected.
The description of wall deposition at the
extreme apex of the hypha, given in Sect. V., indeed
Apical Wall Biogenesis 65
indicates that the nature of the wall in that region
differs completely from that in subapical regions; it
can easily be envisaged that the assumed gel-like,
highly hydrated condition of the wall at the extreme
apex facilitates the diffusion of large proteins over
this barrier. However, the measurements of Money
(1990) on solute exclusion in living hyphae do not
support this contention.
An alternative is offered by the bulk-flow hypothesis
proposed by Wessels (1990, 1993, 1999),
which assumes that proteins secreted at the very
tip are pushed through the wall from the inside to
the outside by the accretion of plastic wall polymers
during wall growth. A theoretical consideration on
how a wall volume travels through an expanding
wall at the apex is given by Green (1974). Considering
the model for apical wall growth discussed
in Sect. V., it is plausible that proteins extruded
into the wall will be carried by the flow of plastic
wall material (Fig. 4.3). In general, if the gradient
of extrusion of a particular protein were similar
to the gradient in wall synthesis, then this protein
would be expected to become evenly distributed
throughout the wall. However, if its gradient of extrusion
were steeper, then it would be preferentially
located in the outer wall region (Fig. 4.3). Across
the wall, this region is oldest, most rigidified and
most stretched. Consequently, the wall may have
larger pores in this region than at the inside, and
Fig. 4.3. Schematic presentation of the “bulk-flow” hypothesis
for protein translocation through the wall in a growing
fungal hypha. Proteins contained in secretion vesicles
which fuse with the plasma membrane at the most apical
site are transported by the flow of nascent wall polymers
to the outside of the subapical wall and, if not anchored to
the wall, can then easily diffuse into the medium. Proteins
fromvesicleswhichfusemoresubapicallywiththeplasma
membrane become trapped in the inner portion of the wall