Calmodulin (CaM) is the major component of
calcium signaling pathways mediating the
action of various effectors. Transient increases
in the intracellular calcium level triggered by a
variety of stimuli lead to the formation of
Ca2+/CaM complexes, which interact with and
activate target proteins. In the present study
the role of Ca2+/CaM in the regulation of the
ligand-dependent activation of the epidermal
growth factor receptor (EGFR) has been
examined in living cells. We show that addition
of different cell permeable CaM antagonists to
cultured cells or loading cells with a Ca2+
chelator inhibited ligand-dependent EGFR
auto(trans)phosphorylation. This occurred also
in the presence of inhibitors of protein kinase
C, CaM-dependent protein kinase II and
calcineurin, which are known Ca2+- and/or
Ca2+/CaM-dependent EGFR regulators,
pointing to a direct effect of Ca2+/CaM on the
receptor. Furthermore, we demonstrate that
down-regulation of CaM in conditional CaM
knock out cells stably transfected with the
human EGFR decreased its ligand-dependent
phosphorylation. Substitution of six basic
amino acid residues within the CaM-binding
domain (CaM-BD) of the EGFR by alanine
resulted in a decreased phosphorylation of the
receptor and of its downstream substrate
phospholipase C¿1. These results support the
hypothesis that Ca2+/CaM regulates the EGFR
activity by directly interacting with the CaMBD
of the receptor located at its cytosolic
juxtamembrane region.