Interaction of Yeast Importin α with the NLS of Prothymosin α Is Insufficient to Trigger Nuclear Uptake of Cargos

Abstract

A proliferation-related human protein prothymosin α displays exclusively nuclear localization when produced in human and Saccharomyces cerevisiae cells, whereas its isolated bipartite NLS confers nuclear targeting of the GFP reporter in human but not in yeast cells. To test whether this observation is indicative of the existence of specific requirements for nuclear targeting of proteins in yeast, a set of prothymosin α deletion mutants was constructed. Subcellular localization of these mutants fused to GFP was determined in yeast and compared with their ability to bind yeast importin α (Srp1p) in vitro. The NLS of prothymosin α turned out to be both necessary and sufficient to provide protein recognition by importin α. However, the NLS-importin α interaction did not ensure nuclear targeting of prothymosin α derivatives. This defect could be complemented by adding distinct prothymosin α sequences to the NLS-containing import substrate, possibly by providing binding site(s) for additional components of the yeast nuclear import machinery.