A purified cellular extract accelerates the cell cycle in Physarum polycephalum

Nuclear translocation of constitutive heat shock protein 70 during S phase in synchronous macroplasmodia of Physarum polycephalum

The level of constitutive heat shock protein 70 (HSC70) in Physarum polycephalum was analyzed by means of Western blots during the synchronous cell cycle of macroplasmodia. Total amounts as well as nuclear and cytoplasmic contents were determined separately and evaluated densitometrically. A drastic increase of nuclear HSC70 was observed 10-40 min after the initiation of S phase (600% of the M phase value) and thereafter a slow decline toward the next M phase. Total HSC levels showed a slight (30%) increase during S phase whereas cytoplasmic HSC70 was about 30% lower during S phase compared to mitosis.

G1 shortening following unbalanced growth: a specific v. nonspecific effect

The synthesis and abundance of proteins were examined in synchronous populations of HeLa cells under conditions in which the lengthening of S phase, by inhibiting DNA synthesis, resulted in shortening of G1 in the subsequent generation. Mitotically collected cells were resynchronized by incubation with 3 microM aphidicolin from 3 to 12 h after mitotic selection; they were blocked again at various times thereafter to induce unbalanced growth. Cells were labelled with [35S]-methionine before and after release from the block to study the changes in protein synthesis. Triton X-100 soluble and insoluble proteins were analysed by 7-15% gradient SDS-PAGE, and radioactivity incorporation was quantified by liquid-scintillation counting. The degree of G1 shortening correlated with S phase position, increasing gradually from early S and reaching maximum when cells were blocked half-way through S phase. Synthesis of proteins of 120, 66, and 51 kDa was stimulated, and synthesis of a new protein of 57kDa was observed, in cells in which DNA synthesis had been blocked in mid-S. These proteins also showed increased accumulation. These results suggest that the shortening of G1, induced by the prior arrest of cell-cycle progression, is associated with synthesis of specific proteins rather than the non-specific accumulation of cellular proteins through unbalanced growth.