Consequences of polyamine acetylation and export in bacteria

Abstract

Polyamines are the natural organic cations of the cell. They are found in all organisms and are known to be required for optimal cell growth and viability. Here it is shown that restoration of polyamine levels in polyamine deficient HT653 cells restores growth. An important mechanism of polyamine degradation in animal cells occurs through acetylation. This is the rate limiting step in the degradation pathway. Polyamine acetylation has been shown to be activated when the cell is presented with a stressful environment. It is believed that acetylation acts to inactivate and deplete the functional concentration of polyamines in the cell. The suppressed polyamine levels result in growth cessation and a potential mechanism in which cells may survive hostile environments. This hypothesis implies that the acetyl-derivatives in themselves have no affect on growth and are simply degradative intermediates. Polyamine acetylation deficient CAG2242 cells over expressing the human polyamine acetyltransferase, N¹SSAT have suppressed polyamine levels, an abundance of the acetyl-derivatives, and exhibit suppressed growth. Restoring polyamines to their original levels with exogenous addition does not restore growth. This gives evidence that the acetyl-derivatives confer a suppressive effect on growth and are in fact not biologically inert. The levels of polyamines within a cell are under tight regulation through mechanisms of synthesis, degradation, import, and export. Recently, a multidrug transporter in Bacillus subtilis, Blt, has been characterized. This protein is co-transcribed and co-regulated with a protein, BltD, that has high homology to a variety of N-acetyltransferases. Our work shows that Blt mediates efflux of spermidine out of the cell. In addition, BltD specifically acetylates both spermidine and spermine at the propylamine moieties. The fact that these proteins are co-regulated in the same operon strongly suggests that this represents a mechanism for rapid polyamine depletion. Interestingly, cells over expressing Blt and BltD grow at slower rates and are sporulation deficient. The mammalian N¹SSAT and Escherichia coli SpeG polyamine acetyltransferases have been characterized in detail. Here, the polyamine acetylation properties of BltD are characterized. Comparisons are made to the mammalian and E. coli counterparts and models are proposed.