Cell proliferation-dependent expression of two isoforms of the nucleolar phosphoprotein p130.

A highly phosphorylated human nucleolar protein p130 (130-kDa) was found to be expressed in synchrony with cell-growth activation. It was not detectable in the resting lymphocytes, but its expression was increased rapidly after mitogenic stimulation. During the terminal differentiation-coupled growth-arrest of HL60 cells, the mRNA and protein of p130 reduced significantly within 24 h after the induction. In addition to the previously identified form (now referred to as p130 alpha ), a novel isoform p 130 beta was found. It contains an insert of ten amino acids located within a region corresponding to the fourth proline-rich basic domain of p130 alpha. Both isoforms were coexpressed in cell lines of different origins, with the beta-transcript exhibiting much less compared to the alpha-transcript. Furthermore, both alpha- and beta-transcripts diminished when cells returned to the quiescent stage. cDNA transfection experiments demonstrated that the two p130 isoforms existed stably in the nucleoli and showed the same nucleolar distribution pattern. It implies that the ten-amino-acid-insert does not alter significantly the interactions of p130 with other nucleolar components in interphase cells.