Distinct Types of Abnormality in Kinetic Properties of Three Darier Disease-causing Sarco(endo)plasmic Reticulum Ca2+-ATPase Mutants That Exhibit Normal Expression and High Ca2+ Transport Activity

Abstract The possible functional abnormalities in three different Darier disease-causing Ca2+-ATPase (SERCA2b) mutants, Ile274 → Val at the lumenal end of M3, Leu321 → Phe on the cytoplasmic part of M4, and Met719 → Ile in P domain, were explored, because they exhibited nearly normal expression and localization in COS-1 cells and the high ATPase and coupled Ca2+ transport activities that were essentially identical (L321F) or slightly lower (I274V by ∼35% and M719I by ∼30%) as compared with those of the wild type. These mutations happened to be in Japanese patients found previously by us. Kinetic analyses revealed that each of the mutants possesses distinct types of abnormalities; M719I and L321F possess the 2–3-fold reduced affinity for cytoplasmic Ca2+, whereas I274V possesses the normal high affinity. L321F exhibited also the remarkably reduced sensitivity to the feedback inhibition of the transport cycle by accumulated lumenal Ca2+, as demonstrated with the effect of Ca2+ ionophore on ATPase activity and more specifically with the effects of Ca2+ (up to 50 mm) on the decay of phosphoenzyme intermediates. The results on I274V and M719I suggest that the physiological requirement for Ca2+ homeostasis in keratinocytes to avoid haploinsufficiency is very strict, probably much more than considered previously. The insensitivity to lumenal Ca2+ in L321F likely brings the lumenal Ca2+ to an abnormally elevated level. The three mutants with their distinctively altered kinetic properties will thus likely cause different types of perturbation of intracellular Ca2+ homeostasis, but nevertheless all types of perturbation result in Darier disease. It might be possible that the observed unique feature of L321F could possibly be associated with the specific symptoms in the pedigree with this mutation, neuropsychiatric disorder, and behavior problems. The results also provided further insight into the global nature of conformational changes of SERCAs for ATP-driven Ca2+ transport.