Importin types

Simple basic NLS SV40 type a stretch rich in basic residues and often in proline residues Importin /3/importin a family... 

Truant R, Cullen BR. The arginine-rich domains present in human immunodeficiency virus type 1 TAT and Rev function as direct importin beta-dependent nuclear localization signals. Mol Cell Biol 1999 19(2) 1210-1217. 

In keeping with the similarity In function of importins and exportins, the two types of transport proteins are highly homologous In sequence and structure. The entire family Is called the Importin p family, or karyopherins. There are 14 karyopherins In yeast and more than 20 In mammalian cells. The NESs or NLSs to which they bind have been determined for only a fraction of them. Remarkably, some Individual karyopherins function as both an Importin and an exportin. 

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