Table 1.

Selected adapter proteins for nuclear transport receptors (NTRs)

Adapter	Protein architecture	Link to NTR pathway	Recycling (NTR)	References
Importin alpha (protein family of ten members in Arabidopsis)	N-terminal IBB domain, 9-10 Armadillo (ARM) repeats; two Arabidopsis proteins differ from this architecture	Importin beta	Exportin 2/CAS	Görlich et al. (1994, 1995a, b, 1996b)
Snurportin 1	N-terminal IBB domain, SPN or TMG-binding domain	Importin beta	Exportin 1	Huber et al. (1998); Paraskeva et al. (1999)
NMD3	NMD3 family domain, NLS, NES	Exportin 1	Importin alpha/beta	Ho et al. (2000); Thomas and Kutay (2003); Trotta et al. (2003)
PHAX	NLS, NES, PHAX RNA-binding domain	Exportin 1	Importin alpha/beta	Ohno et al. (2000)

Only Importin alpha proteins have been characterised in plants to date. Importin alpha is the cytoplasmic receptor for most proteins containing a basic NLS. Other selected nuclear transport adapter proteins: Snurportin 1 is the import adapter for trimethylguanosine (TMG)-capped U snRNPs, NMD3 functions as export adapter for large ribosomal subunits, and PHAX (phosphorylated adapter for RNA export) functions in U snRNA export. In the Arabidopsis genome, a single gene encoding a protein with high sequence similarity to each of the human adapters Snurportin 1, NMD3, and PHAX exists

IBB Importin beta-binding; NLS nuclear localisation signal; NES nuclear export signal