CALCIUM-BINDING AND ZINC-BINDING PROTEINS IN INTRACELLULAR-TRANSPORT

The complex mechanism of intracellular transport is regulated by free calcium in different manners. Calcium binding proteins regulate severa l aspects of the vesicle fusion mechanism mediated by NSF (N-ethylmale imide sensitive fusion factor). At least in some regulated exocytosis, calcium-binding proteins are the trigger for fusion downstream of NSE Still, calcium-binding proteins, such as annexins, may be part of a d ifferent fusion mechanism mediating some specific transport steps or w orking in parallel to the NSF-dependent fusion process. Calcium is not the only ion necessary for the function of factors involved in vesicu lar transport. A zinc requirement has been also proposed. One of the z inc-dependent factors is probably a protein with a cysteine-rich regio n that coordinates zinc and binds phorbolesters. Although protein kina se C is the more prominent family of proteins carrying this domain, th e factor necessary for transport does not appear to function as a kina se.