cal increases in cytoplasmic concentration that underlie the broad range of cellular functions mediated by calcium signaling. Ca2+ entry into the cell or release into the cytoplasm, its extrusion or uptake, are broadly described in the calcium transport interactive pathway diagram - click here to access it directly , (or on the diagram to get to its ontology report). The molecules/classes involved in the many aspects of Ca2+ transport are integral components of the 'calcium signaling kit' and thus present in the current diagram. The synopsis describes the sensors and buffers, the receptor and channels that respond to/are activated by Ca2+; these molecules/classes are highlighted in the diagram. Ca2+, mostly a second messenger, interacts with a broad range of calcium binding, calcium-sensing molecules. Binding of Ca2+ triggers conformational changes that allow the sensors to interact with and modulate the activity of their target effectors.

By far the most prominent calcium sensor is calmodulin (CaM) - a small and ubiquitous molecule conserved throughout evolution that regulates the activity of a large array of targets employing several interaction modes. CaM has a dumbbell-like structure whose lobes house two EF-hand motifs, Ca2+ binding sites with distinct affinities and kinetics for the metal ion. In addition, its interaction mode with targets can modulate the apparent affinity for calcium. Notable CaM targets include calmodulin kinases and calcineurin. Calcineurin is a serine-threonine phosphatase whose Ca2+/CaM dependent signaling activity results in activation of transcription factors and changes in gene expression important for immune and neuronal functions. CaM regulates voltage operated channels (VOCs): the Ca2+ dependent negative feedback of L-type Cacna1c (Cav1.2) is accomplished via CaM; the P/Q type Cacna1a (Cav2.1) on the other hand exhibits Ca2+CaM dependent facilitation and inactivation. In both cases the CaM lobes play distinct roles. CaM in its apo- (Ca2+ free) form acts as a partial agonist of ryanodine receptors (RYRs), whereas the Ca2+ bound sensor acts as an inhibitor. CaM is also a constitutive component of several protein complexes, including the Ca2+ activated K+ channels, discussed further in this synopsis.

Other sensors include synaptotagmins, the neuronal calcium sensor proteins (NCS), troponin C, and members of the CaBP and S100 families of EF-hand containing proteins, only present in vertebrates. Synaptotagmins of which Syt1 is the best studied member, are transmembrane proteins that employ a C2 Ca2+ binding domain; metal coordination is provided by both the protein and membrane lipids. The two C2 domains of Syt1 bind three and two Ca2+ ions, respectively. Syt1 appears to be involved in synaptic vesicle exocytosis and is localized in presynaptic terminals. Proposed Ca2+ sensors, important for the secretion of insulin-containing granules in the pancreatic beta cells are Syt7 and 9. Of note is the fact that several steps underlie vesicular exocytosis; synaptotagmins are believed to be involved in the Ca2+ dependent interaction with the SNARE complex, the main engine of membrane fusion. However, both synaptotagmins and the SNARE complex, along with other proteins probably function in other exocytic steps, such as docking and/or priming.

The NCS family comprises 14 members with four EF-hands motifs, of which some may not be functional. Members, such as guanylate cyclases and recoverin, are only found in the retina. Other members include the widely expressed and better studied Ncs1, the less well characterized visin-like proteins (VSNLs: hippocalcin, neurocalcin delta, and Vilip 1-3) and the four K+ channel- interacting proteins (KChIPs). KChIPs associate with transient voltage-gated potassium channels and stimulate their traffic to the plasma membrane and depending on member identity may carry out other functions as well.

There are two troponin C genes, skeletal and cardiac, each comprising four EF-hands. Troponin, which is composed of three gene products - troponin C, I and T, exerts a major role in the regulation of excitation-contraction coupling. Troponin, in association with tropomyosin, confers the Ca2+ sensitivity of actomyosin underlying the role Ca2+ play in the regulation of muscle contraction.

Of the CaBP family, Cabp1 and 4 are the better studied system and appear to be involved in the regulation of Ca2+ channels. Long and short splice variants of CaBP1 and 2 lead to subtle differences in their localization; functional information is available for the long CaBP1 isoform. Two additional proteins known as calneuron 1 and 2 might represent a subclass of CaBP.

The S100 family - a relatively new comer to the sensor list whose members exhibit a canonical EF-hand and a 'pseudo EF-hand' unique to this family which in some cases, has lost the ability to bind Ca2+. Some family members are only present in human while others appear to be common to both human and rodents with S100a1 present in heart and S100b in the brain. Other S100 proteins can be found in the heart but in lower amounts; for instance S100A4 with which S100A1 can form a heterodimer and S100A1 also heterodimerizes with S100B. Some S100 proteins also bind zinc and copper; depending on the identity of the protein, Zn2+ may either increase or decrease the affinity for Ca2+ binding.

Potassium and chloride channels are calcium sensitive/calcium activated channels. Ca2+ entry via various channels or released from intracellular stores, activates the small conductance K+ channels (SK, Kcnn1-3). SKs don't directly bind Ca2+; when Ca2+ binds CaM, constitutively associated with the intracellular CaM binding domain, highly conserved across the family, the resulting conformational changes lead to opening of the channel's gate. Casein kinase acts on closed channels to decrease Ca2+ sensitivity while protein phosphatase 2A acts on opened ones to increase sensitivity, via phosphorylation and dephosphorylation of SK-associated CaM. The two enzymes are constitutively bound to Sk2 and 3 (Kcnn2 and 3). Models predict multiple contacts on the intracellular termini of the pore-forming subunits for the kinase; one domain C-terminal positioned with respect to the CaM binding site for the phosphatase. The channels are expressed throughout the nervous system and play important roles in synaptic plasticity, learning and memory. Much less is known about calcium activated chloride channels. Despite name/nomenclature, CLCA is an unlikely Ca2+ activated chloride channel (family members related to the bovine protein first purified, have been cloned). A protein known as TMEM16A (known as ANO1) may be the bona fide Ca2+ activated Cl- channel; more work needs to be done to unveil the function and mechanism of TMEM16A and whether the other TMEM16 family members share them (not shown).

In what may be the only instance of a first messenger role, Ca2+ binds to and activates the calcium sensing receptor (Casr), a G protein coupled receptor that triggers several intracellular cascades in a cell-specific manner. The pathway plays a prominent role in calcium homeostasis by regulating the expression and secretion of parathyroid hormone (Pth) and renal calcium reabsorption.

Another class of Ca2+ binding proteins is represented by buffers whose role is to chelate and therefore regulate the concentration of available metal ion. Unlike sensors, buffers do not undergo conformational changes; however, some might and as such could potentially also act as sensors. Cytosolic buffers are represented by parvalbumin alpha and beta (the function of beta/oncomodulin is not well known), calbindin - with the 28K Calb1 and the 9K S100g proteins, and calretinin; all members of the EF-hand family. The organellar ER buffer is calreticulin which also acts as a molecular chaperone together with calnexin and disulfide isomerase Pdia3, known as ERp57. In the SR, calsequestrins provide the buffering function with Casq1 in skeletal and Casq2 in cardiac muscle, respectively.

The regulated availability of the free metal ion and the interaction with its many sensors, the movement of Ca2+ in and out of cell and organelles, the calcium-mediated signaling and calcium transport pathways are inextricably connected. Together, they are components of the 'calcium signaling kit' and together, they orchestrate the homeostasis of what is probably the most versatile element in the kingdoms of life. To see the ontology report for annotations, Gviewer and download, click here . [Click to see associated KEGG map map04020]...(less)