Figure 4.
A model for the roles of calcium in GTP- and calmodulin-dependent nuclear protein import. Under resting cell conditions, the calcium concentration is high in the lumen of the nuclear envelope and endoplasmic reticulum and low in the cytoplasm. Nuclear transport of nucleophiles is stimulated by Ran/TC4 in the GTP-bound state (Left). When the cell is activated, calcium is released from the lumen of the nuclear envelope and endoplasmic reticulum. Evidence indicates that this release of lumenal calcium may inhibit the GTP-dependent activation of transport. However, the elevated cytoplasmic calcium interacts with calmodulin, and the activated calmodulin (CaM·Ca2+) stimulates nuclear import (Right).