CHAPTER 1 INTRODUCTION
1.6 Calmodulin
1.6.1 Calmodulin is a conserved Ca2+ sensor protein
Calmodulin is a small protein with the molecular weight of around 16 kDa. Calmodulin is
an acidic protein. It has four EF-hand motifs, and Ca2+ is bound by those EF-hand motifs.
Calmodulin consists of two symmetrical domains connected by a long α-helix. Two Ca2+ ions
bind to each domain. The long helix involves the interaction of calmodulin with many proteins
(Motohashi 1991).
Calmodulin is expressed in all eukaryotic cells and is highly conserved among different
species. Calmodulin is a typical Ca2+ sensor, as the conformation of calmodulin will change in
the presence of Ca2+ (Ikura 1996). Specifically, the structure of calmodulin can be changed from
random coil (without Ca2+) to a dumbbell shape structure (with Ca2+). Calmodulin is able to
respond to an intracellular Ca2+ concentration change. The range of intracellular Ca2+
concentrations is 10-7M to 10-6M in most cells. The affinity of calmodulin for Ca2+ falls in the
range of 5x10-7M to 5x10-6M, which matches well with the range of intracellular Ca2+
concentration.
The total amount of calmodulin (10-6M to 10-5M) in cells constitutes over 0.1% of the
total proteins. The expression level of calmodulin can be upregulated in rapidly growing cells
1.6.2 Calmodulin binding proteins
Calmodulin itself has no intrinsic enzymatic activity, while it has the capability to
regulate biological activities of many cellular proteins as well as transmembrane proteins.
Calmodulin binding proteins can be classified into at least six groups based on their model of
regulation in the presence of absence of Ca2+. In group I, the effectors bind to calmodulin
irreversiblly in a Ca2+ independent manner. However, the effector/calmodulin complex may be
activated in the presence of Ca2+. In group II, the effectors bind to calmodulin in the absence of
Ca2+, but dissociate in the presence of Ca2+. In group III, the effectors form low affinity and
inactive complexes with calmodulin at low concentration of Ca2+; while at high concentration of
Ca2+, these effectors engage in a high-affinity and active complex with calmodulin. In group IV,
the effectors bind to calmodulin in the presence of Ca2+, and calmodulin inhibits their function.
In group V, effectors are activated by Ca2+-calmodulin, which is a more conventional behavior.
In group VI, effectors bind to Ca2+-calmodulin. The formed complex will be further regulated by
another calmodulin-regulated kinase (Chin and Means 2000).
P68 is known to interact with calmodulin. Interestingly, two groups showed that the
interaction between p68 and calmodulin is Ca2+ dependent, and another showed that this
interaction is Ca2+ independent. However, the functional relevance of the interaction is unknown.
In vitro experiments showed that the RNA-dependent ATPase of p68 was regulated by the
binding to calmodulin.
1.6.3 Calmodulin translocation in the cells
The concentration and location of calmodulin appear to play important roles in regulating
its biological activity. The local concentration of calmodulin may have significant influence as
(10-12~10-16M) (Chin and Means 2000). Interestingly, the majority of calmodulin is observed to be freely diffusible, as shown in the experiment with serum-deprived Swiss 3T3 fibroblasts.
Calmodulin is immobilized under the stimulation of serum (Gough and Taylor 1993). Most
calmodulin is Ca2+ bound in unstimulated smooth-muscle cells. This information indicates that
calmodulin is not always freely diffusible inside the cells. Mechanism that regulates the
calmodulin translocation may exist. Understanding the mechanism by which the calmodulin
translocates in the cells is important, as translocation of the calmodulin appears to be required for
many of its biological functions.
Interestingly, calmodulin is observed to be redistributed from the cytosol to the nucleus in
response to a rise of Ca2+ in cells (Luby-Phelps, Hori et al. 1995). The movement of calmodulin
to the nucleus is also observed in the neuron cells under the stimulation (Deisseroth, Heist et al.
1998). In hormone-treated pancreatic acinar cells, calmodulin is accumulated slowly in the
nucleus (Craske, Takeo et al. 1999). Due to the small size of the calmodulin (16.8 kDa),
calmodulin has a similar pattern of accumulation in the nucleus with 20 kDa fluorescein-dextran.
It is speculated that accumulation of calmodulin in the nucleus is the result of the passive
diffusion of calmodulin into the nucleus (Liao, Paschal et al. 1999). However, whether all the
calmodulin nucleus translocation depends on the passive diffusion is not conclusive.
Interestingly, Richard Thorogate and co-workers found that the majority of calmodulin nuclear
entry occurs by the facilitated mechanism in both Ca2+ dependent and Ca2+ independent manners
(Thorogate and Torok 2004). Since calmodulin has important function in the cytosol, the nuclear
calmodulin needs to be redistributed to the cytoplasm and to a specific area. However, the
mechanism governing this redistribution is unclear. The diffusion as well as the redistribution of
crowding and cytoskeleton. The elevated Ca2+ can also slow down the diffusion of calmodulin in the presence of calmodulin kinase II (Kim, Heinze et al. 2004). Therefore, redistribution of
calmodulin by active mode seems very likely.
1.7 PKM2
1.7.1 PKM2 and tumor
Cancer cells employ glycolysis to generate energy and phosphointermediates which is
different from the normal tissue. Glycolysis is a metabolic pathway that converts glucose into
pyruvate and ATP. Glycolysis involves ten reactions. There are three regulation points in the
pathway: hexokinase, phosphofructokinase, and pyruvate kinase. Pyruvate kinase (PK) is the
third rate-limiting enzyme in glycolysis. The transfer of phosphoryl group from
phosphoenolpyruvate (PEP) to ADP to produce pyruvate and ATP is catalyzed by pyruvate
kinase. There are four isoforms of pyruvate kinases (L type, R type, M1 type, and M2 type) in
mammalian cells. PKM2 is one isoform of pyruvate kinase which is expressed in highly
proliferating tissues including fetal tissue and tumor tissues. During tumorigenesis, M2 type of
pyruvate kinase is expressed while other tissue specific form of PK is completely down-
regulated (van Berkel, de Jonge et al. 1974; Miwa, Nakashima et al. 1975; Etiemble and Boivin
1976; Marie, Kahn et al. 1976). The expression of M2 type of pyruvate kinase can change the
glycolysis rate to produce more phosphoryl intermediates for cell growth.
PKM2 has three structural forms: monomer, dimer, and tetramer. Each monomer of
PKM2 has four domains: A domain, B domain, C domain and N domain. Two monomers bind to
each other at the A domain interface to form a dimeric structure. Two dimers form a tetramer
through binding at the A domain and C domain interface (Dombrauckas, Santarsiero et al. 2005).
constructed by A domain and B domain. The FBP binding site for PKM2 is positioned at the C-
domain. Lysine 433 at the C-domain is important for the binding of FBP to PKM2, and this site
is important for PKM2 binding to the tyrosine phosphorylated peptides (Christofk, Vander
Heiden et al. 2008; Spoden, Morandell et al. 2009). Tetrameric PKM2 has a higher affinity for its
glycolysis substrate PEP than dimeric PKM2 does.
Interestingly, there are more dimeric PKM2 in cells during tumorigenesis. The existence
of high portion of dimeric PKM2 is reasoned to be beneficial for cell growth and proliferation.
This is because the low affinity of dimeric PKM2 to PEP decreases the glycolysis rate, and more
phosphor intermediates is then produed. The phosphor intermediates can be used to synthesize
lipids, amino acids and some other macromolecules which are essential for cell proliferation.
Studies show that pp60V-src kinase and HPV-16E7 proteins directly interact with PKM2,
and help the conversion of dimeric form from tetrameric form (Zwerschke, Mazurek et al. 1999).
However, how the tetrameric PKM2 converts to dimeric PKM2 is not well studied. Interestingly,
PKM2 is a phosphor tyrosine binding protein, and the binding of these peptides can decrease its
enzymatic activity (Christofk, Vander Heiden et al. 2008).
1.7.2 Localization of PKM2
As a pyruvate kinase, PKM2 is dominantly located in the cytoplasm. Interestingly, PKM2
is also found in the nucleus under certain situations. Under the stimulation of interleukin 3 (IL-3),
more PKM2 localizes to the nucleus to regulate cell proliferation in hematopoietic cell, Ba/F3
(Hoshino, Hirst et al. 2007). PKM2 interacts with several nuclear proteins, such as Oct-4 and
PIAS3 (Lee, Kim et al. 2008). Sumoylation changes protein properties including transcription,
nuclear-cytosolic localization as well as protein stability. The overexpression of PIAS3 increases
Morandell et al. 2009). Somatosatin, a neuropeptide hormone and its anti-cancer analogues, TT-
232, interact with pyruvate kinase in COS-7 cells. The interaction induces the nuclear
accumulation of PKM2 from the cytoplasm to trigger cell apoptosis (Stetak, Veress et al. 2007).
Moreover, UV and H2O2 cause PKM2 nuclear accumulation independent of its kinase activity to
induce cell apoptosis (Stetak, Veress et al. 2007). Even though the nuclear location of PKM2 has
been observed, the putative function of nuclear PKM2 is a paradox. Furthermore, the mechanism
in regulating the translocation of PKM2 to the nucleus is not known.