ABSTRACT
The complex mechanism of intracellular transport is regulated by free calcium in different manners. Calcium binding proteins regulate several aspects of the vesicle fusion mechanism mediated by NSF (N-ethylmaleimide sensitive fusion factor). At least in some regulated exocytosis, calcium-binding proteins are the trigger for fusion downstream of NSF, Still, calcium-binding proteins, such as annexins, may be part of a different fusion mechanism mediating some specific transport steps or working in parallel to the NSF-dependent fusion process. Calcium is not the only ion necessary for the function of factors involved in vesicular transport. A zinc requirement has been also proposed. One of the zinc-dependent factors is probably a protein with a cysteine-rich region that coordinates zinc and binds phorbol esters. Although protein kinase C is the more prominent family of proteins carrying this domain, the factor necessary for transport does not appear to function as a kinase.