Metabolic regulation of protein-bound glutamyl phosphates: Insights into the function of prothymosin alpha

Prothymosin alpha gene expression accompanies growth of all mammalian cells . The protein, which is abundant, exceedingly acidic, and localized to the nucleus, is further distinguished by the presence of clustered phosphorylat ed glutamic acid residues (Trumbore et al., 1997, J Biol Chem 272:26394-264 04). These glutamyl phosphates are energy rich and unstable in vivo and in vitro (Wang et al., 1997, J Biol Chem 272:26405-26412). To understand the f unction of prothymosin alpha in greater detail, the turnover of its phospha tes was examined in metabolically manipulated cells. Phosphate half-lives i n growing, mock transfected, and vector-transfected COS cells were compared with the half-life in cells transfected with the prothymosin alpha gene to determine the fate of the predominantly ectopic phosphorylated protein. Th e values obtained-72-75 min in cells with normal levels of the protein, but 118 min in cells with surplus prothymosin alpha-led us to conclude that un derutilized phosphates persist whereas functioning phosphates disperse. Cel l-cycle-specific differences in the half-lives were observed in NIH3T3 cell s: 72 min while cycling, 83 or 89 min during arrest in or progression throu gh S phase, but 174 min during M-phase arrest. In the presence of actinomyc in D, the value was about 145 min regardless of whether cells were quiescen t or growing. In these experiments, reduced utilization of prothymosin alph a's glutamyl phosphates, signaled by an increase in their half-lives, accom panied the attenuation or abolition of transcription. Our data suggest that prothymosin a fuels an energy-requiring step in the production, processing , or export of RNA. I Cell Physiol 178:154-163, 1999. Published 1999 Wiley- Liss, Inc.dagger.