Analysis of G-Quadruplex Formation in mRNA Transcripts of Phospholemman/FXYD1

G-quadruplexes are higher-order nucleic acid structures formed by tetrads of guanine bases (G-tetrads) through non-canonical base interactions. Two G-tetrads are stabilised by a potassium-ion sandwiched between the tetrads. It has emerged from recent studies that G-quadruplexes occur widely throughout the human genome and have significant biological roles. In this study the FXYD1 pre-mRNA encoding the protein Phospholemman (PLM) is investigated. PLM is highly expressed in cardiomyocytes and forms a third subunit of the Na+/K+ pump (NKA). PLM is a major phosphorylation target and thus regulates NKA activity. FXYD1 pre-mRNA was investigated for its ability to form G-quadruplexes. By computational analysis, it was found that FXYD1 can fold into G-quadruplex and multiple sequence alignment of ortholog FXYD1 sequences indicated that G-quadruplex-forming potential is conserved in evolution, hinting at a potential regulatory mechanism of FXYD1 expression. Comparative analysis confirmed that FXYD1-009, a variant of FXYD1, is a product of alternative splicing of FXYD1’s pre-mRNA. G-quadruplex formation in human and bovine FXYD1-derived oligonucleotides was detected experimentally by non-denaturing poly acrylamide gel electrophoresis that showed an increased mobility rate of G-quadruplexes in contrast to controls. Further analysis by fluorescence emission spectroscopy confirmed G-quadruplex formation in the human and bovine FXYD1-oligonucleotides that was triggered by the presence of K+ ions. The results provided clear evidence of G-quadruplex formation in vitro and together with evolutionary conservation point to potential role in regulating expression of FXYD1 possibly through alternative splicing and thus regulate indirectly the activity of Na+/K+-ATPase. Further in-vivo works should address whether alternative splicing of FXYD1 to FXYD1-009 is associated with G-quadruplex formation

G-quadruplex formation of FXYD1 pre-mRNA indicates the possiblity of regulating expression of its protein product

G-quadruplexes are higher-order nucleic acid structures formed of square-planar arrangements of four guanine bases held together by Hoogsteen-type hydrogen bonds. Stacks of guanine tetrads are stabilised by intercalating potassium ions. FXYD1 encodes for phospholemman, a regulatory subunit of the cardiac Na+/K+-ATPase. Computational sequence analysis of FXYD1 pre-mRNA predicted the formation of stable intramolecular G-quadruplexes in human and orthologue sequences. Multiple sequence alignment indicated that G-rich sequences are conserved in evolution suggesting a potential role of G-quadruplexes in FXYD1 gene expression. The existence of a non-functional alternative splicing product indicated that the G-quadruplex formation may control alternative splicing. Quadruplex formation of human and bovine oligonucleotides was confirmed in vitro by native polyacrylamide gel electrophoresis and intrinsic fluorescence emission spectroscopy. Taking together the evolutionary conservation of G-quadruplex forming sequences with the confirmation of G-quadruplex formation in vitro by two FXYD1 homologues the results point to a potential role of these structures in regulating the expression of FXYD1 and thus regulate indirectly the activity of the cardiac Na+/K+ -ATPase.Peer reviewe