Advances in Na⁺,K⁺‐ATPase studies: from protein to gene and back to protein

Abstract

Complete primary stators of both submit of Na',K+-ATPase from various sources have been ~~biisbed by a ~mbination of the methods for molecular cloning and protein chemistry. The gene family homologo~ to the a-subunit cDNA of animal Na+,K*-ATPases has been found in the human genome. Some genes of this family encode the known isoforms (aI and aI1) of the Na+,K+-ATPase catalytic subunit. The proteins coded by other genes can be either new isoforms of the Na+,K+-ATPase catalytic subunit or other ion-transporting ATPases. Expression of the genes of this family proceeds in a tissue-specific manner and changes during the postnatal development and neoplastic transformation. The complete exon-intron structure of one of the genes of this family has been established. This gene codes for the form of the catalytic subunit, the existence of which has been unknown. Apparently, all the genes of the discovered family have a similar intron-exon structure. There is certain correlation between the gene structure and the proposed domain a~angement of the a-subunit. The results obtained have become the basis for the experiments which prove the existence of the earlier unknown afI1 isoform of the Na+,K+-ATPase catalytic subunit and have made possible the study of its function.

The genes of Na, K-ATPase, a selfreview

“…As previously noted (Ovchinnikov et al, 1988;Broude et al, 1989), the phosphorylated and nucleotide-binding domains are separated by introns 9, 10 and I 1 (see Fig. 2) whereas the latter domain is cut by introns 12, 13 and 14 into two subdomains.…”

“…This allowed us to compare the three sequences and conclude that they are highly conservative , the homology being over 90%. In the few years that followed more ~ subunit cDNAs of Na,K-ATPases were sequenced and their high homology with one another was confirmed even for such evolutionarily remote animals like insects and mammals (Lingrel et al, 1989;Broude et al, 1989;Lebowitz et al, 1989;Serrano, 1990;Briskin, 1990). A calculation of the divergence rate constant for the a subunit yields the Kaa (amino acid substitution) value of 0.18× 10 -9 implying that this protein belongs to the most evolutionarily conservative ones, between cytochrome C and histone H4.…”

Mineralocorticoid Hormone Signaling Regulates the ‘Epithelial Sodium Channel’ in Fibroblasts from Human Cornea

“…Corneal deturgescence is apparently assured by an endothelial barrier to the bulk passage of water and solutes from the aqueous humor in the anterior chamber to the stroma. This is believed to be regulated by the Na + -K + -ATPase in the lateral plasma membrane of corneal endothelium [1][2][3][4], although no experimental proof is available to support this hypothesis, and the thermodynamic properties of the enzyme do not corroborate this idea [5][6][7]. The colocalization of the MCR and the ENaC, in corneal endothelium, suggests that sodium could actually be absorbed by the ASSC present at the apical end of the cell facing the anterior chamber and extruded into the stroma by the sodium pump at the basolateral end lining Descemet's membrane.…”

“…First, sodium ion has been causally linked to the expression and maintenance of several important functions in the eye, including that of the phototransduction cascade [1][2][3][4]. The notion that ocular sodium transport may be assured by the basolateral Na + -K + -ATPase [1][2][3][4] is not in keeping with the thermodynamic and other properties of the enzyme [5][6][7]. Second, circumstantial evidence suggests that steroid hormones play an important role in ocular sodium ion transport [14][15][16].…”

“…The Na + -K + -ATPase is actually an enzyme whose primary structure is highly conserved and which is ubiquitously expressed in the basolateral membrane in a cell-and tissue-specific manner. The intracellular portion of the enzyme is endowed with sites that bind sodium and expel the cation into the extracellular compartment in exchange for potassium that binds to the extracellular portion of the enzyme and is consequently pumped into the cell [5][6][7]. Thus, the basolateral Na + -K + -ATPase cannot be expected to perform a regulatory role for sodium uptake from the extracellular space which is generally in contact with the apical membrane of the cell [8,9].…”