Nuclear phosphoproteins of Physarum polycephalum. Characterization and phosphorus content of the phenol-soluble nuclear acidic proteins

Polyamine-activated protein kinase reaction from nuclei and nucleoli of Physarum polycephalum which phosphorylates a unique Mr 70 000 nonhistone protein

Methods are described for the detection and purification of a protein kinase from nuclei and nucleoli of Physarum polycephalum which catalyzed transfer of phosphate from [gamma-32P]ATP to a unique nonhistone protein of Mr 70 000 in a reaction that was polyamine dependent. Enzymatic phosphorylation of the nonhistone protein by the purified protein kinase was stimulated greatly, at times more than 60-fold, by the polyamines spermidine and spermine. This unique polyamine-dependent reaction was localized on the rDNA minichromosome of the nucleolus. The polyamine-dependent protein kinase, which was first partially purified with the acidic nonhistone protein fraction from isolated nucleoli, was resolved from at least six other protein kinases by phosphocellulose chromatography into a catalytic component of Mr 26 000 and a complex comprised of the catalytic component associated with a phosphate acceptor protein of Mr 70 000. The complex also catalyzed polyamine-dependent phosphorylation of the endogenous Mr 70 000 component. The resolved catalytic component catalyzed polyamine-dependent phosphorylation of a dephosphorylated Mr 70 000 nonhistone protein that had been independently isolated from nucleoli and previously demonstrated to have properties concordant with a specific regulatory role in rRNA gene transcription [Keuhn, G. D., Affolter, H. U., Atmar, V. J., Seebeck, T., Gubler, U., & Braun, R. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2541-2545]. These studies indicate one way that the polyamines may regulate rRNA gene transcription through the mediation of a highly specific nonhistone protein kinase.

Phosphorylation of nuclear and DNA-binding proteins in proliferating and quiescent mammalian cells

The dependence of cell proliferation on nuclear protein phosphorylation was studied with exponential-phase and stationary-phase cultures of Chinese-hamster ovary cells. Nuclear proteins were fractionated, according to their DNA-binding affinities, by using sequential extractions of isolated nuclei with increasing concentrations of NaCl. When viable whole cells were labelled with H332PO4, phosphorylation of nuclear proteins was found to be lower in quiescent cells than in proliferating cells. Phosphorylation of nuclear proteins soluble in 0.30M-NaCl (less than 50% of these proteins bind to DNA) was greater than for those proteins soluble in higher salt concentrations (80-100% of these proteins bind to DNA). Cyclic AMP enhanced the phosphorylation of nuclear proteins soluble in 0.3 m-NaCl by 40-50%, and this stimulation was independent of cell growth. Cyclic AMP also increased the phosphorylation of nuclear proteins soluble in 0.6M-NaCl and 2.0M-NaCl by 40-50% in exponential-phase cultures, but not in stationary-phase cultures. Several examples of specific phosphorylation in response to cyclic AMP were observed, including a 35000-mol.wt. protein in the 0.30 M-NaCl-soluble fraction and several proteins larger than 100000 molecular weight within this fraction. A major peptide of molecular weight approx. 31000 extracted with 0.6M-NaCl was also phosphorylated. Its phosphorylation was independent of cyclic AMP in exponential-phase cultures, and it was not phosphorylated in plateau-phase cells. These changes in cell-growth-dependent phosphorylation occurred in the absence of any apparent qualitative changes in the nuclear protein molecular-weight distributions. These data demonstrate that (1) phosphorylation of nuclear proteins is dependent on the culture's proliferative status, (2) both cyclic AMP-dependent and cyclic AMP-independent specific phosphorylation occurs, and (3) the cyclic AMP-dependent growth-independent phosphorylation that occurs does not appear to be a modification of DNA-binding proteins, whereas the cyclic AMP-dependent growth-dependent phosphorylation does involve modification of DNA binding proteins.

Early changes in the synthesis of proteins with affinity for single-stranded DNA during the onset of transformation in NRK cells

Early alterations in the synthesis of proteins which bind to single-stranded DNA have been examined following the onset of transformation in NRK cells transformed by a heat-sensitive mutant (ts339) of Rous sarcoma virus. Transformation was initiated by shifting quiescent cultures from nonpermissive to permissive temperatures. Cultures were prelabelled with [3H]leucine for several generations at the non-permissive temperature, and with [35S]methionine at times after shift to the permissive temperature. Cytosol extracts were passed through sequential columns of double-stranded and single-stranded DNA bound to cellulose. Within the first hour of transformation there was an increase in the synthetic rate of proteins binding tightly to single-stranded DNA, but not to double-stranded DNA. More loosely bound protein fractions showed no such early synthetic increase. Electrophoresis of the fraction eluted from single stranded DNA-cellulose with 2 M NaCl demonstrated the presence of a major protein of 93 000 daltons, which comprised more than 0.1% of the cytosol protein. The synthesis of the 93 000 dalton protein increased continuously over the first 4 h interval after the onset of transformation. The synthetic rate of a 35 000 dalton protein, a major DNA-binding polypeptide found in mammalian cells, began to increase after a 1-h lag, following the onset of transformation. The protein fraction containing the 93 000 dalton protein had considerable unwinding activity, depressing the melting temperature of poly(dA-dT) by 39 degrees C. The protein fraction containing the bulk of the 35 000 dalton protein did not have unwinding activity. Transformation-induced DNA synthesis was measured in cells made permeable to deoxyribonucleoside triphosphates at times after shift to the permissive temperature. It was determined that synthesis of DNA began within the first 1--2 h after the onset of transformation. We conclude that the early transformation-associated synthesis of SS93 and perhaps other proteins binding to single-stranded DNA may be related to early transformation-associated changes preparatory to DNA replication and subsequent growth.