The Isoform-Specific Region of the Na,K-ATPase Catalytic Subunit:  Role in Enzyme Kinetics and Regulation by Protein Kinase C

Abstract

Comparisons of the primary structures of the Na,K-ATPase R-isoforms reveal the existence of regions of structural divergence, suggesting that they are involved in unique functions. One of these regions is the isoform-specific region (ISR), located near the ATP binding site in the major cytoplasmic loop. To evaluate its importance, we constructed mutants of the rodent wild-type R1 and R3 isoforms in which the ISR was replaced with irrelevant sequences, i.e., the analogous region from the rat gastric H,K-ATPase catalytic subunit or a region from the human c-myc oncogene. Opossum kidney (OK) cells were transfected with wild-type rat R1, R3, or their corresponding chimeras and selected in ouabain. Introduction of either mutant produced ouabain-resistant colonies, consistent with functional expression of the chimeric protein and indicating that the ISR is not essential for overall Na,K-ATPase function. The introduced chimeras were then characterized enzymatically by measuring the relative rate of K + and Li + deocclusions. Results showed that exchanges of both R1 and R3 ISRs significantly modified the sensitivity for the enzyme to either K + or Li + . Subsequent treatment of the cells with phorbol esters revealed an altered Na,K-ATPase transport in response to protein kinase C activation for the R1 chimeras. No changes were observed for the R3 isoform, suggesting that it is not sensitive to PKC regulation. These results demonstrated that the ISR plays an important role in ion deocclusion and in the response to PKC (only for the R1 isoform).The Na,K-ATPase, which catalyzes the extrusion of Na + and the absorption of K + at the expense of metabolic energy derived from ATP, consists of at least two subunits, R (110 kDa) and (55 kDa), existing as several isoforms. The R subunit, representing the catalytic component of the enzyme, is a membrane-spanning protein expressed as four isoforms (R1, R2, R3, and R4) with differences in enzyme kinetics and response to second messengers (1). Comparisons of the primary structures of the rat isoforms have revealed regions of structural divergence that could be involved in isoformspecific functions. A major site of sequence divergence among the R isoforms is the isoform-specific region (ISR 1 ; Figure 1) (2). This 11-amino acid sequence is located in the major cytoplasmic loop between TM4 and TM5 near the ATP binding site (2). Alignment and homology comparison of the rat R ISR amino acid sequences (Figure 1) reveal significant diversity among the isoforms. Indeed, while the rat R1 ISR shares 27% of homology with the R4 ISR, the R1 and R2 ISRs, as well as the R1 and R3 ISRs, share only 9% of their residues.Because of its proximity to the ATP binding site, we speculated that the replacement of the ISR by unrelated sequences would alter the enzymatic properties of the molecule. In a previous study, we have shown that the R1 ISR is essential in protein kinase C (PKC) regulation (2). The present study aims to investigate further the importance of the Na,K-ATPase R1 ISR and to des...

Critical Role of the Isoform-Specific Region in α1-Na,K-ATPase Trafficking and Protein Kinase C-Dependent Regulation

“…We examined the likely position of the ISR, taking advantage of the recent high-resolution structure obtained for Na,K-ATPase complex from pig kidney (22). Homology modeling of the rat sequence against this structure suggests that the ISR of α protrudes into the cytoplasm from the N or “nucleotide-binding” domain, consistent with earlier models based on the sarcoplasmic reticulum Ca 2+ -ATPase (15) (Fig 6B). At least theoretically, this places the ISR in an accessible position for interaction with intracellular partners such as clathrin APs.…”

Time-dependent increases in ouabain-sensitive Na+, K+-ATPase activity in aortas from diabetic rats: The role of prostanoids and protein kinase C

“…In line with these results, we only observed increased phosphorylation of PKC βII in the diabetic aorta, suggesting a major role of this isoform of PKC in the aorta of type I diabetic rats. It has been demonstrated that activation of PKC may stimulate Na + , K + -ATPase activity (Lowndes et al 1990;Pedemonte et al 1997;Duran et al 2004). Gupta et al (1991) demonstrated that endothelin increases Na + , K + -ATPase activity by a PKC-dependent pathway.…”

“…It is wellestablished that Na + , K + -ATPase activity can be modulated by insulin, protein kinase C (PKC) as well as by endothelial factors such as nitric oxide (NO) and prostaglandins (Lowndes et al 1990;Gupta et al 1991;Skou and Esmann 1992;Pedemonte et al 1997;Marin and Redondo 1999;Duran et al 2004). It is therefore possible that the insulin resistance and endothelial dysfunction characteristic of diabetes could interfere with the activity of Na + , K + -ATPase (Simmons and Winegrad 1993;Sweeney and Klip 1998), although at present this theory remains controversial.…”

Mechanisms Underlying Rapid Aldosterone Effects in the Kidney

“…Structural differences between the α-Na + /K + -ATPase isoforms contribute to differences in their mechanism of regulation. For example, α1 and α3 differ at sequences proximal to the ATP-binding site so that α1 is activated by PKC whereas α3 is not (85). Ca 2+ -dependent isoforms PKCα and PKCβ mediate the phosphorylation of α-Na + /K + -ATPase at its proline-rich motif (86), whereas PKA-dependent phosphorylation occurs at Ser943.…”