Calcium-binding protein

Many different calcium-binding proteins exist, with different cellular and tissue distribution and involvement in specific functions. Calcium binding proteins also serve an important physiological role for cells. The most ubiquitous Ca2+-sensing protein, found in all eukaryotic organisms including yeasts, is calmodulin. Intracellular storage and release of Ca2+ from the sarcoplasmic reticulum is associated with the high-capacity, low-affinity calcium-binding protein calsequestrin. Calretinin is another type of Calcium binding protein weighing 29kD. It is involved in cell signaling and shown to exist in neurons. This type of protein is also found in large quantities in malignant mesothelial cells, which can be easily differentiated from carcinomas. This differentiation is later applied for a diagnosis on ovarian stromal tumors. Also, another member of the EF-hand superfamily is the S100B protein, which regulates p53. P53 is known as a tumor suppressor protein and in this case acts as a transcriptional activator or repressor of numerous genes. S100B proteins are abundantly found in cancerous tumor cells causing them to be overexpressed, therefore making these proteins useful for classifying tumors. In addition, this explains why this protein can easily interact with p53 when transcriptional regulation takes place. Calcium-binding proteins can be either intracellular and extracellular. Those that are intracellular can contain or lack a structural EF-hand domain. Extracellular calcium-binding proteins are classified into six groups. ==Functions==

With their role in signal transduction, calcium-binding proteins contribute to all aspects of the cell's functioning, from homeostasis to learning and memory. For example, the neuron-specific calexcitin has been found to have an excitatory effect on neurons, and interacts with proteins that control the firing state of neurons, such as the voltage-dependent potassium channel. Compartmentalization of calcium binding proteins such as calretinin and calbindin-28 kDa has been noted within cells, suggesting that these proteins perform distinct functions in localized calcium signaling. It also indicates that in addition to freely diffusing through the cytoplasm to attain a homogeneous distribution, calcium binding proteins can bind to cellular structures through interactions that are likely important for their functions. ==See also==