Nuclear distribution of prothymosin alpha and parathymosin: Evidence that prothymosin alpha is associated with RNA synthesis processing and para-thymosin with early DNA replication

Prothymosin alpha and parathymosin are two ubiquitous small acidic nuclear proteins that are thought to be involved in cell cycle progression, prolife ration, and cell differentiation. In an effort to investigate the molecular function of the two proteins, we studied their spatial distribution by ind irect immunofluorescence labeling and confocal scanning laser microscopy in relation to nuclear components involved in transcription, translation, and splicing. Results indicate that both proteins exhibit a punctuated nuclear distribution and are excluded by nucleoli. The distribution of prothymosin alpha in the nucleus is related to that of transcription sites, whereas th e distribution of parathymosin correlates with early replication sites. Thi s implies that prothymosin Lu and parathymosin are involved in transcriptio n and replication, respectively. In addition to the punctate distribution, prothymosin alpha also is found concentrated in 1-6 nuclear domains per cel l. These domains are found in more than 80% of randomly growing T24 human b ladder carcinoma cells. They have a diameter of 0.2-2.5 mu m, their size be ing inversely related to the number of domains per cell. The domains disapp ear during mitosis and the protein is excluded from the metaphase chromosom es. Double-labeling experiments associate these prothymosin alpha domains w ith PML and CstF64 containing nuclear bodies, but not with hnRNP-I containi ng domains or coiled bodies. (C) 2000 Academic Press.