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the case of eukaryotic tubulin, the assembly of FtsZ into filaments can be
mediated by accessory proteins such as ZipA. This prokaryotic protein in partic-
ular is considered to resemble typical MAPs (Amos et al.
2004
). In addition,
direct interaction between FtsZ and enzymes involved in glucose metabolism the
glucosyltransferase UgtP occurs when the intracellular concentration of
UDP-glucose is high and leads to a partial inhibition of division that is thought
to constitute metabolic sensing (Weart et al.
2007
). Direct interaction between
FtsZ and thioredoxin, which helps maintain the intracellular redox potential
(along with its moonlighting activities), may also permit metabolic sensing
(Kumar et al.
2004
); such interaction occurs in chloroplasts too (Balmer
et al.
2003
). The bacterial actin, MreB, which also interacts with thioredoxin,
forms a helix that changes its location depending on FtsZ (Figge et al.
2004
).
Interactions also occur during cell division between FtsZ and a dozen other
proteins involved, for example, in the synthesis of lipids and peptidoglycan
(Norris et al.
2007
). Intriguingly, production in a bacterium of S100B, a
human protein that undergoes a calcium-dependent conformational change to
bind to tubulin, results in its colocalization with FtsZ and inhibition of cell
division (Ferguson and Shaw
2004
).
Specificity is an important criterion in evaluating the physiological relevance of
enzyme interactions; specific recognition of one protein by another is based on their
surface complementarity. This recognition may depend on the conformational state of
both partners as influenced by ligands and by additional macromolecules. The ensem-
ble of these effects on interactions determines the functions of both the microtubule
system and the proteins/enzymes involved in the metabolic and signaling pathways.
The associations of the glycolytic kinases with the microtubular system are
oligomeric and isoform specific (Partikian et al.
1998
), e.g., in the case of the
dissociable muscle PFK isoform, the inactive dimeric form of the enzyme binds to
MTs whilst in the case of brain the active tetrameric form does not bind (Verkman
2002
) (Fig.
7.1
). Some of these interactions are modulated by key metabolites and
nucleotides, like fructose-phosphates and ATP (W´gner et al.
2001
) indicating that
the intra- and intermolecular forces between enzyme subunits and enzyme/MT,
respectively, might control the dynamism and superstructure of the microtubular
network in neuronal or mitotic cells. Such dynamic macromolecular associations
could constitute an innovative control mechanism for energy production via gly-
colysis. In addition, the involvement of these associations in many physiological
functions of the cell makes it likely that when they are perturbed pathological, e.g.,
neurodegenerative, processes result.
7.2 Moonlighting Proteins Display Multiple Functions
Decoding the human genome has made it clear that the straightforward
“one gene—
one enzyme”
law of classical molecular biology is inadequate since many more
proteins have been identified than protein-coding sequences which account for only
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