Relationship between intron 4b splicing of the rat geranylgeranyl diphosphate synthase gene and the active enzyme expression level

Geranylgeranyl diphosphate synthase (GGPS) is a branch point enzyme in the mevalonate pathway that catalyzes the synthesis of geranylgeranyl diphosphate used for the geranylgeranylation of Rho, Rac and Rab proteins. The current study showed the production of multiple forms of GGPS mRNA from a single GGPS gene in rat. The mRNAs resulted from combinations of multiple alternative introns and two poly(A) sites in the 3′-translated and 3′-untranslated regions. These are classified into 1a-type and 1b-type mRNAs, based on the splicing of intron 4b resulting in the difference in deduced amino acid sequence between the C-terminal regions. The 1a-type and 1b-type proteins expressed in both Escherichia coli and HeLa cells were active and inactive, respectively. In the case of HeLa cells, the latter protein expression level was about 10% relative to the former one. This was also observed for Cos-7 and 293 cells. When fusions of β-galactosidase with C-terminal regions differing between the 1a-type and 1b-type proteins were expressed in HeLa cells, the expressed fusion proteins were both found to be active but the latter fusion protein expression level was considerably low compared with the former one. The expression level of 1a-type mRNA was higher than that of 1b-type mRNA in brain, liver, heart, and thymus, but the two expression levels were the same in testis and ovary. During testis development the total GGPS mRNA expression level increased, accompanied by an increase in 1b-type mRNA, the expression level of 1a-type mRNA encoding active GGPS remaining kept unchanged. These results indicate that the expression level of rat active GGPS is at least regulated through the splicing of intron 4b of its gene.