The calcium- and zinc-responsive regions of calreticulin reside strictly in the N-/C-domain

Calreticulin (CRT) is a chaperone of the endoplasmic reticulum. We dissected CRT into its two structural domains, N-/C-domain and P-domain, to identify its metal ion-responsive region. For this, we constructed bacterial expression systems for the N-/C-domain (1–180 fused by a linker to 290–400) and P-domain (189–280). Circular dichroism (CD) studies showed that calcium ions increased tertiary packing and thermal stability of apo N-/C-domain, whereas zinc ions had a strong destabilizing effect. Interestingly, neither calcium nor zinc ions altered the structural properties of apo P-domain. These results indicate that the calcium- and zinc-responsive regions reside strictly in the N-/C-domain. Analysis of thermal denaturation curves of CRT, N-/C-domain, and P-domain suggested a structural role for the P-domain in CRT. Rotary shadowing electron microscopy (EM) analysis of CRT and calnexin provided convincing evidence for their structural relatedness. This analysis also revealed that apo P-domain adopts various curved shapes suggesting conformational flexibility. EM images of apo N-/C-domain revealed objects having wide gaps suggesting weak interactions between the N- and C-domains. This is consistent with the larger size of apo N-/C-domain on the gel filtration column. Our studies provide a framework for correlating the structural organization of CRT with its metal ion-responsive region.