Identification of a novel S6K1 splice variant coding for the p60-S6K1 isoform

Aim. To identify a splicing mRNA of S6K1 kinase specific for the p60-S6K1 isoform alone. Methods. RT-PCR, DNA sequencing, Western blotting. Results. RT-PCR analysis of total RNA extracted from MCF-7 cells and subsequent DNA sequencing revealed a novel S6K1 splice variant that possesses additional exon 1a and encodes the p60 isoform of S6K1 only. Moreover, RT-PCR and western blotting were applied to estimate the expression levels of the p60-S6K1 mRNA and protein. A heterogeneous expression of the p60-S6K1 transcript was observed; it did not correlate with the total S6K1 mRNA expression and with the protein content of p60-S6K1 in the studied cell lines. Conclusions. We provide the evidence for the existence of a novel S6K1 splice variant coding for the p60-S6K1 isoform. The cell can employ two distinct pathways of the p60-S6K1 expression based on alternative translation of mRNA common for the main S6K1 isoforms mRNA and/or translation of the novel p60-S6K1 specific mRNA. Since the levels of the p60-S6K1 transcript expression do not correlate with the expression of total S6K1 mRNA, it is plausible that the p60-S6K1 isoform plays a role distinct from that of other isoforms in cellular physiology, as well as in the development of S6K1-related pathologies.