The EF-Hand, Homologs and Analogs
Many proteins bind calcium; they have different structures and employ different coordination geometries to achieve a wide range of affinities and selectivities. Kretsinger (1975) proposed that those within the cytosol are calcium modulated, that they are in the magnesium or apoform in the quiescent cell and in the calcium form in the stimulated cell, and that they detect calcium functioning as a second messenger. Further, he proposed that these calcium modulated proteins, in contrast to the vast array of extracytosolic calcium-binding proteins, all contain the EF-hand homolog domain and are all members of one homolog family. The generality of this theory should be questioned. What exactly is a quiescent cell? How do pores and pumps figure in this scheme? Do some proteins lacking EF-hands, such as the annexins, bind messenger calcium? Do some EF-hand proteins have a high enough affinity to bind calcium in quiescent cells? Do some EF-hand proteins function in the extracytosolic environment? Do these generalizations apply to prokaryotic cells? Although we will address some of these questions, our main concern here is to identify and characterize the members of the EF-hand homolog family and to distinguish them from several very similar analogs.
KeywordsMold Influenza Proline Progesterone Cytosol
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