Chlorpromazine binding to Na+, K+-ATPase and photolabeling: involvement of the ouabain site monitored by fluorescence

Chlorpromazine and dimethyl sulfoxide modulate the catalytic activity of the plasma membrane Ca2+-ATPase from human erythrocyte

The plasma membrane Ca²⁺-ATPase (PMCA) removes Ca²⁺ from the cytosol into the extracellular space. Its catalytic activity can be stimulated by calmodulin (CaM) or by limited proteolysis. We evaluated the effect of chlorpromazine (CPZ) and dimethyl sulfoxide (DMSO) over the hydrolytic activity of PMCA. Activity was monitored in three different forms: native, CaM-activated and proteolyzed by trypsin. CPZ appears to inhibit PMCA without directly interfering with the C-terminal site, since it is affected by CaM and proteolysis. Although the treatment of PMCA with trypsin and CaM produces an activation, it also produces an enzymatic form that is more sensitive to inhibition by CPZ. The same case was observed in the DMSO inhibition experiments. In the absence of CPZ, DMSO produces a progressive loss of activity, but in the presence of CPZ the profile of activity against DMSO changes and produces a recovery of activity, indicating a possible partition of CPZ by the solvent. Increasing Ca²⁺ concentrations indicated that CPZ interacts with PMCA rather than with CaM. This observation is supported by docking analysis that suggests that the CPZ-PMCA interaction is non-competitive. We propose that CPZ interacts with the state of lower affinity for Ca^{2 +}.

Critical role of the isoform-specific region in alpha1-Na,K-ATPase trafficking and protein Kinase C-dependent regulation

The isoform-specific region (ISR) is a region of structural heterogeneity among the four isoforms of the catalytic alpha-subunit of the Na,K-ATPase and an important structural determinant for isoform-specific functions. In the present study, we examined the role of a potential dileucine clathrin adaptor recognition motif [DE]XXXL[LI] embedded within the alpha1-ISR. To this end, a rat alpha1 construct where leucine 499 was replaced by a valine (as found in the alpha2 isoform sequence) was compared to wild-type rat alpha1 after stable expression in opossum kidney cells. Total Na,K-ATPase expression, activity, or in situ (86)Rb(+) transport was not affected by the L499V mutation. However, surface Na,K-ATPase expression was nearly doubled. This increase was associated with a reduced rate of internalization from the cell surface of about 50% after a 4 h chase and became undetectable if clathrin-coated pit-mediated trafficking was blocked with chlorpromazine. Further, PKC-induced stimulation of Na,K-ATPase-mediated (86)Rb(+) uptake was doubled in mutant-expressing cells, comparable to the chimera containing the intact alpha2-ISR. Similar results were observed when the potential motif was disrupted by means of an E495S mutation. These findings suggest that a dileucine motif embedded within the Na,K-ATPase alpha1-ISR plays a critical role in the surface expression of Na,K-ATPase alpha1 polypeptides at steady state and in the response to PKC activation.