
- •Calcium-binding by proteins
- •Calmodulin and troponin C
- •Kinases regulated by calmodulin
- •Calcium-dependent enzymes that are not regulated by calmodulin
- •Paradigms of calcium signalling
- •Triggering neurotransmitter secretion
- •Initiation of contraction in skeletal muscle
- •Smooth muscle contraction
- •References

Chapter 8
Calcium effectors
Calcium-binding by proteins
In the previous chapter we have seen how transient elevations of the concentration of free Ca2 may occur in cells in response to receptor activation or membrane depolarization. We also saw how these signals may be confined at subcellular locations or may propagate and how they may be temporally encoded as spikes or oscillations. Now we ask how changes in [Ca2 ] within the cytosol are sensed and converted into downstream signals. Not surprisingly, Ca2 -binding proteins are involved. However, molecules that bind Ca2 are not necessarily mediators of signalling. For instance, cytosol proteins with very high Ca2 affinity will already be saturated and will remain unaffected by a rise in its concentration above the resting level (40–100 nmol L 1). Lower affinity proteins with dissociation constants 0.1 mol L 1 will certainly bind more Ca2 , but again it does not necessarily follow that they will take part in signalling. Instead, they may be important as buffers that stabilize Ca2 levels or help to shape Ca2 transients. On the other hand, there is a diversity of signalling proteins that bind Ca2 at regulatory sites and that are activated by increases in its level. These are the Ca2 effectors. A selection is listed in Table 8.1.
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Table 8.1 Examples of effector proteins of vertebrate origin that are activated by Ca2
Protein |
Type |
Ca2 -binding |
Function |
Main location |
|
|
domains |
|
|
|
|
|
|
|
Calreticulin |
|
|
Ca2 buffering |
ER and SR |
|
|
|
|
|
Calsequestrin |
|
|
Ca2 buffering |
SR in muscle |
|
|
|
|
|
Parvalbumin |
|
EF hand |
cytosolic Ca2 buffering |
muscle/nerve |
|
|
|
|
|
Ca2 -ATPase (PMCA) |
CaM dependent |
|
pumps Ca2 out of cell |
plasma |
|
|
|
|
membrane |
|
|
|
|
|
Ca2 -ATPase (SERCA) |
|
|
pumps Ca2 into stores |
ER membrane |
|
|
|
|
|
Calmodulin (CaM) |
|
EF hand |
multipurpose Ca2 sensor |
|
|
|
|
|
|
Troponin C |
|
EF hand |
Ca2 -sensor mediating |
striated muscle |
|
|
|
contraction |
|
|
|
|
|
|
Calmodulin kinases I, II, IV |
CaM is a regulatory |
|
multipurpose signalling |
|
|
subunit |
|
|
|
|
|
|
|
|
Myosin light chain kinase |
Ca2 /CaM dependent |
|
phosphorylates myosin II |
smooth muscle |
|
|
|
|
|
Adenylyl cyclases 1, 8 |
Ca2 /CaM dependent |
|
makes cyclic AMP |
|
|
|
|
|
|
Adenylyl cyclases 5, 6 |
|
|
makes cyclic AMP (Ca2 inhibits) |
|
|
|
|
|
|
Transduction Signal

Cyclic nucleotide phosphodiesterase |
Ca2 /CaM dependent |
|
breaks down cyclic AMP |
|
(1A–C) |
|
|
|
|
|
|
|
|
|
Phosphorylase b kinase |
CaM is a regulatory |
|
phosphorylates glycogen |
skeletal muscle |
|
subunit |
|
phosphorylase |
|
|
|
|
|
|
Recoverin |
Ca2+- myristoyl switch |
EF hand |
Ca2+- sensing mediator |
photoreceptor |
|
|
|
|
cells |
|
|
|
|
|
Calpain |
|
EF hand |
protease |
|
|
|
|
|
|
-Actinin |
|
EF hand |
cytoskeleton |
|
|
|
|
|
|
Gelsolin |
|
|
actin severing and capping |
|
|
|
|
|
|
Synaptotagmin |
Putative Ca2 sensor |
C2 |
signalling |
secretory cells |
Calcineurin (protein phosphatase 2B) |
Ca2 /CaM dependent |
EF hand |
signalling, e.g. transcription |
|
|
|
|
|
|
Protein kinase C ( , 1, 2, ) |
|
C2 |
signalling |
|
|
|
|
|
|
Phospholipase C (all isoforms) |
|
EF hand, C2 |
signalling |
|
|
|
|
|
|
Diacylglycerol kinase |
|
EF hand |
makes phosphatidate |
|
|
|
|
|
|
Nitric oxide synthase |
CaM is a regulatory |
|
production of NO for signalling |
|
|
subunit |
|
|
|
CaM, calmodulin; PMCA, plasma membrane Ca2 -ATPase; SERCA, sarco/endoplasmic reticulum Ca-ATPase.
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effectors Calcium |

Signal Transduction
Polypeptide modules that bind Ca2
Common Ca2 -binding sites on proteins include the EF-hand motif and the C2 domain. Their structures are described in Chapter 24. EF-hand motifs occur in pairs and a single motif binds a single Ca2 ion with KD in the range 10 7–10 5 mol L 1. However, some EF-hands have lost or never even acquired the ability to bind Ca2 , for example the EF-hands of phospholipase C (see Chapter 5). The Ca2 dissociation constants of C2 domains also vary widely (10 6–10 3 mol L 1) and again, some do not bind Ca2 at all.
Many Ca2 effectors are without either C2 domains or EF-hands, but acquire their Ca2 dependence through the Ca2 sensor protein calmodulin, which in some cases forms an integral subunit (Table 8.1). Other Ca2 -binding proteins lack C2 domains or EF-hands and function independently of calmodulin. They may be found among the wide range of channels and ATPases that conduct Ca2 ions across membranes. Further examples are the buffering proteins calreticulin and calsequestrin, present in the ER and SR, the extracellular adhesion molecule cadherin, and the actin-modifying protein gelsolin.
Decoding Ca2 signals
When Ca2 signals are transient, an effector must be able detect the change and initiate a response before the concentration subsides. When Ca2 rises transiently, sensing the change does not just depend on the stability constant of the binding, but also the rates of the‘on’and‘off’reactions. The forward reaction requires the successive displacement, one at a time, of water molecules from the solvation shell of the cation. In general, if a multidentate coordinating ligand is to associate with or dissociate from a cation rapidly, then its framework needs to be flexible. The evolution of such coordination sites in proteins has led to the emergence of Ca2 -activated regulatory enzymes that can bind and respond very rapidly to changes in Ca2 concentration.
Ca2 signals are often repetitive, taking the form of trains of spikes or pulses with periods of the order of minutes (see Figure 7.6, page 195) and it is likely that there are specific effectors that can respond to this form of temporal encoding. While a sustained rise in [Ca2 ] might be damaging because it could activate downstream effectors indiscriminately, an oscillatory signal would favour those with low effective off-rates, allowing them to retain their bound Ca2 during a downswing. It would also favour effector pathways that remain active for just long enough to ensure throughput during dips in [Ca2 ]. Evidence is accumulating in support of these ideas. For instance, in T lymphocytes a cytoplasmic Ca2 signal that oscillates can be more effective at activating transcription than one that is steady. Moreover, high-frequency oscillations can activate three different transcription factors, while at low frequencies only NF- B is activated1,2 (see page 521).
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