Ras-induced colony formation and anchorage-independent growth inhibited byelevated expression of Pur alpha in NIH3T3 cells

Levels of Pur alpha, a conserved, sequence-specific single-stranded DNA and RNA binding protein, fluctuate during the cell cycle, declining at the ons et of S-phase and peaking at mitosis. In early G1 Pur alpha is associated w ith the hypophosphorylated form of the retinoblastoma protein, Rb. Microinj ection of purified Pur alpha into NIH3T3 cells arrests the cell cycle at ei ther G1/S or G2/M checkpoints with distinct morphological consequences. Her e we ask whether expression of Pur alpha can affect colony formation and an chorage-independent growth in ras-transformed NIH3T3 cells. Two to five-fol d elevated levels of Pur alpha in stably-transfected cell lines retard entr y into and progression through S phase in both ras-transformed and non-tran sformed cells. Pur alpha significantly inhibits colony formation by ras-tra nsformed cells but not by non-transformed cells. In addition, cells transfe cted to express Pur alpha formed only about 1/5 the number of large colonie s in soft agar as control-transfected cells, demonstrating a marked inhibit ion of anchorage-independent growth by Pur alpha. Biochemical analysis of n uclear and cytoplasmic Pur alpha proteins and confocal microscopic analysis of Pur alpha location indicate that access of Pur alpha to the nucleus is controlled by both protein modification and sequence domains within the pro tein. Analyses of deletion mutants identify Pur alpha domains mediating nuc lear exclusion, including several potential destruction motifs and a PEST s equence at aa's 215-231. In the nucleus Pur alpha colocalizes with CDK2 and cyclin A. Pur alpha and cyclin D1, however, do not colocalize in the nucle us. At mitosis Pur alpha is visualized about the condensed chromosomes and in the cytoplasm, where it colocalizes with cyclin B1. The data indicate th at the ability of Pur alpha to interact with proteins regulating cell proli feration and transformation is controlled by signals that govern its intrac ellular localization. J. Cell. Biochem. 81:621-638, 2001. (C) 2001 Wiley-Li ss. Inc.