The study shows that prothymosin alpha, the larger protein that contains thymosin‑alpha‑1, lives inside the cell nucleus rather than being secreted, and its nuclear entry is driven by a specific tail‑end sequence, not the thymosin‑alpha‑1 part.

Prothymosin alpha is a highly acidic protein which lacks an amino-terminal signal peptide, yet was once thought to be a precursor for thymosin alpha 1, a putative peptide hormone secreted by the thymus. Here, two lines of evidence are presented that strongly implicate prothymosin alpha as a nuclear protein: 1) in COS cells transfected with the human prothymosin alpha gene copious amounts of prothymosin alpha were present in sealed nuclei obtained by treating these cells with cytochalasin B and enucleating them centrifugally. 2) Constructs in which human prothymosin alpha nucleic acid sequences were fused in-frame either near the amino terminus of the beta-galactosidase gene in pCH110 or at the carboxyl terminus, when expressed in COS cells, resulted in nuclear localization of the fusion protein; indirect immunofluorescence in situ was used as the assay. The basic cluster of amino acids at the carboxyl terminus of prothymosin alpha, TKKQKT, has been identified as part of the nuclear targeting signal, whereas the basic cluster of amino acids situated within the thymosin alpha 1 sequence at the amino terminus failed to effect nuclear transport.