Histidine-rich Ca2+-binding protein stimulates the transport cycle of SERCA through a conformation-dependent fuzzy complex.

The histidine-rich Ca2+-binding protein (HRC) stimulates the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) to increase Ca2+-uptake into the lumen. HRC also binds the triadin scaffold in a Ca2+-dependent manner, and HRC tunes both the uptake and release of Ca2+ depending on the concentration in the intracellular Ca2+-stores. We investigated how HRC stimulates SERCA pumping using biochemical and biophysical assays, and show that HRC is an intrinsically disordered protein that binds directly to SERCA via electrostatic interactions. The affinity of the interaction depends on the conformation of SERCA, and HRC binds most tightly in the calcium-released E2P state. This state marks the end of the rate-limiting [Ca2]E1P to E2P transition of SERCA, and suggests that HRC stimulates SERCA by preferentially stabilizing the end point of this transition. HRC remains disordered in the bound state and thus binds in a dynamic, fuzzy complex. The binding of HRC to SERCA shows that fuzzy complexes formed by disordered proteins may be conformation-specific, and use this specificity to modulate the functional cycle of complex molecular machines such as a P-type ATPase.