Calmodulin-binding proteins are, as their name implies, proteins which bind calmodulin. Calmodulin can bind to a variety of proteins through a two-step binding mechanism "conformational and mutually induced fit", where typically two domains of calmodulin wrap around an emerging helical calmodulin binding domain from the target protein. Examples include:
A variety of different ions, including Calcium, play a vital role in the regulation of cellular functions. Calmodulin, a Calcium-binding protein, that mediates Ca2+ signaling is involved in all types of cellular mechanisms, including metabolism, synaptic plasticity, nerve growth, smooth muscle contraction, etc. Calmodulin allows for a number of proteins to aid in the progression of these pathways using their interactions with CaM in its Ca2+ bound conformation or as its Ca2+ -free state. Proteins each have their own unique affinities for CaM, that can be manipulated by the presence or absence of Ca2+ concentrations to allow for the desired release or binding to CaM that determines its ability to carry out its cellular function. Proteins that get activated upon binding to Ca2+ -CaM, include Myosin light-chain kinase, Phosphatase, Ca2+/calmodulin-dependent protein kinase II, etc. Proteins, like neurogranin that plays a vital role in postsynaptic function, however, can only bind to the CaM in ApoCaM via their IQ-domain. Since these interactions are exceptionally specific, they can be regulated through Post-translational modification by enzymes like kinases and Phosphatase to affect their cellular functions. In the case of Ng, it's the synaptic function can be inhibited by the PKC-mediated phosphorylation of its IQ-domain that impedes its interaction with CaM. Cellular functions can be indirectly regulated by CaM, as it acts as a mediator for enzymes that require Ca2+ stimulation for activation. Studies have proven that calmodulin's affinity for Ca2+ increases when it is bound to a calmodulin-binding protein, which allows for it to take on its regulatory role for Ca2+-dependent reactions. Calmodulin, made up of 2 pairs of Ef-hand calcium binding domain 2 separated in different structural regions by an extended alpha helical region, that permits it to respond to the changes in the cytosolic concentration of the Ca2+ ions by taking on two distinct conformations, in the inactive Ca2+ unbound state and active Ca2+ bound state. Calmodulin binds to the targeted proteins via their short complementary peptide sequences, causing a “induced fit” conformational change that alters the calmodulin-binding proteins’ activity as desired in response to the second messenger Ca2+ signals that arise due to changes in the intracellular Ca2+ concentrations. These second messenger Ca2+ signals are transduced and integrated to maintain a homeostatic balance of the Ca2+ ions.
GAP-43 Protein
Found in the nervous system, GAP-43 is a growth-associated protein expressed in high levels during presynaptic developmental and regenerative axonal growth. As a major growth cone component, an increase in GAP-43 concentrations delays the process of axonal growth cones evolving into stable synaptic terminals. All GAP-43 proteins share a completely conserved amino acid sequence that contain a CaM binding domain and a serine residue that can be used to inhibit calmodulin binding upon phosphorylation of Protein kinase C. By possessing these calmodulin-binding properties, GAP-43 is able to respond to PKC activation and release free calmodulin in desired areas. When there are low levels of Ca2+ concentrations, GAP-43 is able to bind and stabilize the inactive Ca2+ -free state of calmodulin, this allows it to absorb and reversibly inactivate the CaM in the growth cones. This binding of the calmodulin to GAP-43 is allowed by the negatively charged CaM electrostatically interacting with the positively charged “pocket” formed in the GAP-43 molecule itself.
