ApoCalmodulin and Ca2+-independent calmodulin-binding proteins

Calmodulin (CaM) is ubiquitous, multi functional calcium (Ca2+) sensor that exists in all eukaryotes. As it has no enzymatic activity, CaM transmits the Ca2+ signal by interacting with CaM-binding proteins (CaMBPs) to function in cellular regulation. In recent years, it has been found that CaM not only signals in the presence of Ca2+, it can also directly bind to target proteins as ApoCaM. Ca2+-independent CaM-binding proteins (ApoCaMBPs) is also an important way to elucidate the mechanism of CaM functions. Recent progresses in studies on animal and plant ApoCaMBPs were summarized. ApoCaM differs from Ca2+-CaM in its tertiary structure. It binds target proteins differently, utilizing different binding motifs such as the IQ motif, noncontiguous binding sites and others. The ApoCaMBPs are a diverse group of proteins including enzymes, transcription activators, as well as cytoskeletal and other membrane proteins, including receptors and ion channels. The overall picture that emerges is that CaM cycles between its Ca2+-bound and Ca2+-free states and in each state binds to different proteins and performs essential functions. Although much of the research focus has been on the roles of Ca2+-CaM binding proteins, the roles of ApoCaMBPs are equally vital but less well understood. Researches on ApoCaM and its binding proteins will make us understand the variety of CaM signaling pathway.