Structural Plasticity Allows Calumenin-1 to Moonlight as a Ca-Independent Chaperone: Pb Enables Probing Alternate Inhibitory Conformation.

Human calumenin-1 (HsCalu-1) is an endoplasmic reticulum (ER) and Golgi-resident Ca-binding protein of the hepta-EF-hand superfamily that plays a vital role in maintaining the cytoplasmic Ca concentration below toxic levels by interacting with Sarco/endoplasmic reticulum Ca-ATPase (SERCA) and ryanodine receptors (RyR), indicating its role in Ca homeostasis in the ER. HsCalu-1 seems to be able to exhibit structural plasticity to achieve its plethora of functions. In this study, we demonstrate that HsCalu-1 acts as a chaperone in both its intrinsically disordered state (apo form) and the structured state (Ca-bound form). HsCalu-1 chaperone activity is independent of Ca and Pb binding attenuating its chaperone-like activity. Incidentally, Pb binds to HsCalu-1 with lower affinity ( = 38.46 μM) (compared to Ca-binding), leading to the formation of a less-stable conformation as observed by a sharp drop in its melting temperature from 67 °C in the Ca-bound form to 43 °C in the presence of Pb. The binding site for Pb was mapped as being in the EF-Hand-234 domain of HsCalu-1, a region that overlaps with the Ca-dependent initiator of its functional fold. A change in the secondary and tertiary structure, leading to a less-stable but compact conformation upon Pb binding, is the mechanism by which the chaperone-like activity of HsCalu-1 is diminished. Our results not only demonstrate the chaperone activity by a protein in its disordered state but also explain, using Pb as a probe, that the multiple functions of calumenin are due to its ability to adopt a quasi-stable conformation.