Importin [alpha] from Arabidopsis thaliana Is a Nuclear Import Receptor That Recognizes Three Classes of Import Signals

Abstract

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.