Page 178 - The Biochemistry of Inorganic Polyphosphates

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Peculiarities of polyphosphate metabolism
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8.10.8 The Effects of Mutations on the Content and Chain
Lengths of Polyphosphate in Yeast
Kornberg and co-workers (Kornberg, 1995, Wurst et al., 1995; Kornberg et al., 1999;
Sethuraman et al., 2001) have made a great contribution to identification of the genes
involved in PolyP metabolism in yeast.
Many genes are probably involved in the regulation of PolyP metabolism in yeast.
The most studied enzymes of PolyP metabolism in yeast are exopolyphosphatases and
endopolyphosphatase (see Chapter 6). The genes of one of the exopolyphosphatases, PPX1
(Wurst et al., 1995), and endopolyphosphatase, PPN1 (Sethuraman et al., 2001), were
disrupted and their effects on the dynamics and chain lengths of PolyPs were studied. The
PPX1-defficient mutant had more PolyP at early growth stages than the parent strain, and
the double PPX1 and PPN1-defficient mutant had approximately three times more PolyP
than the parent strain (Figure 8.25). In addition, both mutants contained mostly medium-
chain-length PolyPs when compared with the predominance of short chains in the parent
strain (Figure 8.26). It was proposed that the loss of viability of the PPN1 mutants may be
a result of accumulation of a large amount of PolyP, which could affect the concentration
of important divalent cations such as Ca 2+ and Mg 2+ in the cells (Sethuraman et al., 2001).
When analysing the involvement of new genes in PolyP metabolism, it should be taken
into account that PolyP accumulation strongly depends on the availability of P i for yeast
cells. In S. cerevisiae, the PHO system includes many genes involved in the P i uptake
(Oshima, 1997) and it is not unexpected that the products of these genes may influence
PolyP accumulation and utilization. DNA microarray analysis was used to identify 22
PHO-regulated genes (Ogawa et al., 2000a). Some of these genes, e.g. PHM1, PHM2,
PHM3 and PHM4, are 32–56 % identical. The PHM3 or PHM4 single mutants and the
PHM1/PHM2 double mutant are deficient in accumulation of P i and PolyP. It is probable
that the proteins encoded by these genes are involved in vacuolar transport (Cohen et al.,
1999). The disruption of another gene, PHM5, gives a phenotype with an essentially long
PolyP with no effect on its content in cells (Ogawa et al., 2000a). The PHM5 protein has
a similarity with the yeast endopolyphosphatase, which was characterized by Kumble and
Kornberg (1996).
The pleiotrophic effect of the genes involved in phosphorus metabolism is not a surprise,
because this element is vital for living organisms. PolyP as a phosphate and energy reserve
maybeinvolvedindifferentregulatoryprocesses,andmutationsinthegenesrelatedtoPolyP
metabolism might influence many aspects of cellular regulation. For example, disruption of
the gene YOL002c results in accumulation of PolyP to a much higher level than in the wild-
type cells. In addition, this mutant shows the induction of many genes involved in fatty acid
metabolism, phosphate-signaling pathways and nystatin resistance (Karpichev et al., 2002).
Two ‘bursts’ in the production of acid-soluble PolyPs were shown to occur during the
growth of some S. cerevisiae strains on a medium containing glucose and galactose under
aerobic conditions (Solimene et al., 1980). The respiratory deficient mutant, however, had
only one PolyP ‘burst’, which indicated that the accumulation of PolyP produced in the first
‘burst’ depended on the active mitochondrial function (Solimene et al., 1980).
Many publications have reported the absence of PolyPs in mutants with disturbed vac-
uolar functions (Westenberg et al., 1989; Beauvoit et al., 1991; Shirahama et al., 1996). It