Importin alpha from Arabidopsis thaliana is a nuclear import receptor that recognizes three classes of import signals

Protein import into the nucleus is a two-step process. In vitro import systems from vertebrate cell extracts have shown that several soluble factors are required. One of these factors is the receptor importin alpha, which binds to nuclear localization signals (NLS) in vitro. We previously cloned an importin alpha homolog from Arabidopsis thaliana (At-IMP alpha) and demonstrated that this protein was not depleted from tobacco (Nicotiana tabacum) protoplasts after permeabilization of the plasma membrane (Hicks et al., 1996). To determine if At-IMP alpha is functional, we used an in vitro NLS-binding assay. We found that At-IMP alpha binding is specific, and the receptor is able to recognize three classes of NLS identified in plants. Purified antibodies to At-IMP alpha were used to determine the in vivo location of importin alpha in tobacco protoplasts. Importin alpha is found in the cytoplasm and nucleus, and it is most highly concentrated at the nuclear envelope. The biochemical properties of nuclear importin alpha and localization studies using purified nuclei demonstrate that importin alpha is tightly associated with the plant nucleus. Moreover, these results suggest that a fraction of nuclear importin alpha interacts with the nuclear pore complex.