A prolactin (PRL)-responsive 3'-end cDNA encoding rat alpha4 phosphoprotein was previously isolated from a rat lymphoma cDNA library. Rat alpha4 is a homologue of yeast Tap42 and is a component of the mammalian target-of-rapamycin (mTOR) signalling pathway that stimulates translation initiation and G1 progression in response to nutrients and growth factors. In the present study, the full-length rat alpha4 cDNA was obtained by 5'-RACE and the 1023 bp open reading frame predicted a 340 amino acid protein of 39.1 kDa. The alpha4 mRNA was expressed in quiescent PRL-dependent Nb2 lymphoma cells deprived of PRL for up to 72 h but expression was downregulated within 4 h of PRL treatment. In contrast, PRL-independent Nb2-Sp cells showed constitutive expression of alpha4 that was not affected by PRL. Western analysis of Nb2 cell lysates or of V5-tagged-alpha4 expressed in COS-1 cells detected a single immunoreactive band of approximately 45 kDa. Enzymatic deglycosylation of affinity-purified 45 kDa alpha4 yielded the predicted 39 kDa protein. Phosphorylation of Nb2 alpha4 was induced by PRL or 2-O-tetradecanoyl-phorbol-13-acetate (TPA) and further enhanced by a combination of PRL and TPA. The Nb2 alpha4 associated with the catalytic subunit of protein phosphatase 2A and localized predominantly in Nb2 nuclear fractions with trace amounts in the cytosol. The immunosuppressant drug rapamycin inhibited proliferation of Nb2 cells in response to PRL or interleukin-2, but had no effect on Nb2-Sp cells. Furthermore, transient overexpression of alpha4 in COS-1 cells inhibited PRL stimulation of the immediate-early gene interferon regulatory factor-1 promoter activity. Therefore, PRL downregulation of alpha4 expression and/or PRL-inducible phosphorylation of alpha4 may be necessary for PRL receptor (PRLr) signalling to the interferon regulatory factor-1 promoter in the Nb2 cells and, furthermore, implicates cross-talk between the mTOR and PRLr signalling cascades during Nb2 cell mitogenesis.