Abstract
The process of nuclear protein transport requires the interaction of several different proteins, either directly or indirectly with nuclear localization or targeting sequences (NLS). Recently, a number of karyopherins alpha, or NLS-binding proteins, have been identified. We have found that the karyopherins hSRP1 and hSRP1alpha are differentially expressed in various leukocyte cell lines and could be induced in normal human peripheral blood lymphocytes. We show that the two karyopherins bind with varied specificities in a sequence specific manner to different NLSs and that the sequence specificity is modulated by other cytosolic proteins. There was a correlation between binding of karyopherins alpha to different NLSs and their ability to be imported into the nucleus. Taken together, these data provide evidence for multiple levels of control of the nuclear import process.
Highlights
Active nuclear transport of proteins with molecular weights greater than 40 – 60 kDa requires at least four different proteins, which act in a sequential manner with karyophilic proteins containing nuclear localization targeting sequences (NLS)1 [1,2,3,4]
We found that karyopherin ␣ is present in both the cytoplasm as well as the nucleus in different nuclear:cytoplasmic ratios, depending on the cell type
The 70Z/3 cells are of mouse origin, there are only two amino acid differences between the mouse pendulin (K2) and human hSRP1␣ (K2) in the sequence we used for raising antibodies [11]
Summary
Active nuclear transport of proteins with molecular weights greater than 40 – 60 kDa requires at least four different proteins, which act in a sequential manner with karyophilic proteins containing nuclear localization targeting sequences (NLS)1 [1,2,3,4]. The proteins involved in NLS binding and transport have been identified. Those proteins that interact directly with the NLS have been termed karyopherins ␣ [7,8,9,10,11]. In this report we have termed hSRP1 and hSRP1␣, K1 and K2, respectively Each of these karyopherins ␣ are capable of binding to NLSs and facilitating nuclear import. Perhaps most important for nuclear protein transport is the targeting sequence or NLS. In this report we show that there are multiple levels of control of nuclear import These control points include the sequence specific binding of karyopherins ␣ to various NLSs and modulation of this interaction by other cytoplasmic proteins. The differential and inducible expression of karyopherins ␣ may play a role in regulating nuclear protein transport
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