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 phosphorylated glutamic acid residues (Trumbore et al., 1997, J Biol Chem 272:26394-26404). 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 function of prothymosin alpha in greater detail, the turnover of its phosphates was examined in metabolically manipulated cells. Phosphate half-lives in 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. The 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 underutilized phosphates persist whereas functioning phosphates disperse. Cell-cycle-specific differences in the half-lives were observed in NIH3T3 cells: 72 min while cycling, 83 or 89 min during arrest in or progression through S phase, but 174 min during M-phase arrest. In the presence of actinomycin D, the value was about 145 min regardless of whether cells were quiescent or growing. In these experiments, reduced utilization of prothymosin alpha's glutamyl phosphates, signaled by an increase in their half-lives, accompanied the attenuation or abolition of transcription. Our data suggest that prothymosin alpha fuels an energy-requiring step in the production, processing, or export of RNA.
Tao
L
L
Section on Genes and Gene Products, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Wang
R H
RH
Enkemann
S A
SA
Trumbore
M W
MW
Berger
S L
SL
D016475:3T3 Cells; D000818:Animals; D019556:COS Cells; D003609:Dactinomycin / Q000494:pharmacology; D005971:Glutamates / Q000378:metabolism*; D006207:Half-Life; D051379:Mice; D008938:Mitosis; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism*; D011485:Protein Binding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D012313:RNA / Q000096:biosynthesis; D016196:S Phase; D013947:Thymosin / Q000031:analogs & derivatives* / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D005971:Glutamates; D019384:Nucleic Acid Synthesis Inhibitors; D010710:Phosphates; D011498:Protein Precursors; C040782:prothymosin alpha; D003609:Dactinomycin; D013947:Thymosin; D012313:RNA