Growth, Differentiation and Sexuality

82 J.P. Latgé and R. Calderone
CHSs does exist in moulds. For example, some of
the CHS genes are specialized for conidiation. It has
been found indeed that at least three of the five A.
nidulans chitin synthase genes tested are regulated
by ABAA, a transcriptional regulator of conidiation
in Aspergillus (Park et al. 2003).
In Wangiella dermatitidis, mutations in chitin
synthases genes led to very different phenotypes
(Wang et al. 1999, 2002). Disruption of WdCHS1
class II genes produces strains which form short
chains of yeast cells. The single WdCHS2 (class I)
and WdCHS3 (class III) mutants show no obvious
phenotype. The class IV WdChs4mutantistheonly
chs mutant in this organism which has a statistically
significantreductioninchitincontent.Inaddition,
the WdCHS4 mutant is hyperpigmented with
melanin, and forms aggregates of yeasts. Mutants
in the WdCHS5 gene also have increased melanin,
and reduced viability in late log and early stationary
phases of growth.
Although no chitin has been found in S. pombe
yeast cell walls (Perez and Ribas 2004), CHS1 and
CHS2 and chitin synthase activity have been detected
in this yeast. Chs1p is necessary for maturation
of the spore wall whereas Chs2p is related
to septum formation (Arellano et al. 2000).
Complementing S. cerevisiae chs mutants with CHS
genes from S. pombe has been achieved by Matsuo
et al. (2004) but not by Martin-Garcia et al.
(2003).
Even though CHS enzymes have been carefully
analysed at the genomic and cellular levels, the biochemical
activity of these enzymes remains poorly
analysed. The reasons for proteolytic activation of
some chitin synthases are unknown, as is the occurrence
of such a phenomenon in vivo.
2. Regulation of Chitin Synthesis
Theregulationofchitinsynthesisinfungiisalsofar
from being completely understood and has been
studied only in yeast. No genes directly involved
in the control of chitin synthase activity I and
II have been described. Four yeast genes CHS4-7
which differ at the sequence level from the catalytic
chitin synthases are involved in the regulation
of chitin synthase III activity (Roncero 2002;
Fig. 5.7).
Chs7p acts as a specific chaperone for Chs3p,
allowing its sorting from the ER. In the absence of
this protein, Chs3p accumulates in the ER, producing
an inactive protein both in vivo and in vitro
(Trilla et al. 1999). Chs7p is unique in the S. cere-
Fig. 5.7. The regulation of chitin synthesis, including both
putative and experimental data. CHS3 requires CHS7 to exit
from the ER, and CHS5 and CHS6 to exit from the Golgi.
Inactive and phosphorylated CHS3 is transported from the
endoplasmic reticulum (ER) to the plasma membrane (PM)
where it is dephosphorylated and activated by CHS4. At the
PM, CHS3 can be also associated with actin/myosin and
septins to direct the synthesis at the chitin septum ring.
More than 50% of CHS3 is stored inactive in early endosome
compartments (chitosomes), and is recruited from
the chitosomes during cell wall stress or glucosamine nutrition.
Retrograde transport of CHS3 is regulated by CHS6
or by the clattering AP1 complex to be stored as an intracellular
inactive pool of CHS3 which is immediately available
to the cell when needed (protein names are in capital letters)
visiae genome, and close homologues have been
described in C. albicans, A. fumigatus and Neurospora
crassa.
Chs5p and Chs6p are Golgi proteins required
for the correct sorting of Chs3p to the membrane
(Santos et al. 1997; Santos and Snyder 1997; Ziman
et al. 1998; Valdivia et al. 2002). These two
proteins are essential for the regulation of CSIII
activity: Chs5p is responsible for the transport of
Chs3p inside chitosomes (early endosomes) to the
membrane where it becomes activated. Chs6p is associated
with the retrograde endocytosis of Chs3p
which is maintained dormant in chitosomes. This
pathway regulates the activity without degradation
of the enzyme. Although the function and localisation
of Chs5p and Chs6p seem to be very similar,
close homologues of the ScCHS5, butnotCHS6
genes have been found in C. albicans, A. fumigatus
and N. crassa. These data suggest that the function
of the CHS6 gene may not be associated directly
with chitin synthesis, since other homologues of
Chsp such as YKR027p are not involved in chitin
synthesis (Roncero 2002). In moulds, the retrograde
activity could be under the control of the
clathrin-associated AP-1 proteins which have been
shown to have a role in yeasts and have orthologues
in moulds (Valdivia and Scheckman 2003).