Calcium-dependent phospholipid-binding protein that plays a role in ERBB2-mediated tumor cell migration in response to growth factor heregulin stimulation. Monomer. Interacts with ERBB2 (preferentially with the tyrosine phosphorylated form); this interaction occurs at the cell membrane and is increased in a growth factor heregulin-dependent manner. Interacts with SHC1; this interaction may mediate the binding of CPNE3 with ERBB2. Interacts with RACK1. Associates to the membrane in a calcium-dependent manner. Translocates to the cell membrane and the nucleus in a calcium- or growth factor heregulin-dependent manner. Colocalizes with the tyrosine phosphorylated ERBB2 form at cell membrane and focal adhesions in a calcium- or growth factor heregulin-dependent manner. Phosphorylated on serine and threonine residues. Belongs to the copine family.