In Silico studies provide new structural insights into trans -dimerization of β 1 and β 2 subunits of the Na + ,K + -ATPase
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The Na + ,K + -ATPase is an electrogenic transmembrane pump located in the plasma membrane of all animal cells. It is a dimeric protein composed of α and β subunits and has a third regulatory subunit (γ) belonging to the FXYD family . This pump plays a key role in maintaining low concentration of sodium and high concentration of potassium intracellularly. The α subunit is the catalytic one while the β subunit is important for the occlusion of the K + ions and plays an essential role in trafficking of the functional αβ complex of Na + ,K + -ATPase to the plasma membrane. Interestingly, the β 1 and β 2 (AMOG) isoforms of the β subunit, function as cell adhesion molecules in epithelial cells and astrocytes, respectively. Early experiments suggested a heterotypic adhesion for the β 2 . Recently, we reported a homotypic trans-interaction between β 2 -subunits expressed in CHO cells. In this work we use In Silico methods to analyze the physicochemical properties of the putative homophilic trans-dimer of β 2 subunits and provide insights about the trans -dimerization interface stability. Our structural analysis predicts a molecular recognition mechanism of a trans -dimeric β 2 -β 2 subunit and permits designing experiments that will shed light upon possible homophilic interactions of β 2 subunits in the nervous system.
Author summary
The adhesion molecule on glia (AMOG) is the β 2 isoform of the β-subunit of the Na + -pump that is localized in the nervous system, specifically in astrocytes. It was shown that it mediates Neuron-Astrocyte interaction, promoting neurite outgrowth and migration during brain development. In recent years we have shown that the ubiquitous β 1 isoform is a homophilic adhesion molecule in epithelia and therefore we hypothesized that β 2 could also interact as a homophilic adhesion protein. In a previous work we show that fibroblasts (CHO) transfected with the human β 2 subunit of the Na + -pump become adhesive. Moreover, protein-protein interaction assay in a co-culture of cells transfected with β 2 tagged with two different markers (His 6 and YFP) reveal a positive interaction between the β 2 -subunits. In the present work, we apply bioinformatics methods to analyze and discuss the formation of a trans -dimer of β 2 -subunits. Our In Silico study predicts a relatively stable dimer with an interface that involves the participation of four out of the seven N-glycosylation sites. Nevertheless, interacting interface and the dynamics of the β 2 -β 2 trans -dimer is different from that of the β 1 -β 1 dimer; it involves different surfaces and therefore it explains why β-subunits can not form mixed (β 1 -β 2 ) trans -dimers.
