The Eliminated Negative Charge at NLS increases the PAK I Binding to Karyopherins
Objective: Pak1 (p21-activated kinase 1) is increasingly linked to cancer. This is surprising since the PAK I proteins, which participate in the actin-cytoskeleton formation, are tightly regulated by the Cdc42 GTPase. However, assaying interactions of Pak1 with β-karyopherins, the nuclear transport factors, suggests that it could cause cancer in the karyopherin-dependent manner. The Pak1-dependent depletion of Kpnβ1 is predicted to strongly increase the requirement for the Kpn β1 expression. In turn, the increased expression of Kpn β1 is predicted to increase nuclear import of the AP-1 transcriptional regulator, cyclin D1 expression, cell proliferation and cancer growth. We propose that increased binding of Pak1 to Kpn β1 initiates vicious circle in which the Pak1-dependent depletion of Kpn β1 is followed by the increased expression of this protein and cell proliferation.
Methods: The mutagenized Ste20 PAK from S. cerevisiae was tested for its ability to bind to karyopherins. The mutant-expression phenotypes were examined to establish the β -karyopherin deficiency.
Results: Ste20 forms protein complexes with two β-karyopherins, Kap121 (Kpn β3) and Kap123 (Kpn β1-like karyopherin). The loss of-phosphorylation mutation (Ser268, Ser269Ala268, Ala269) increases the binding to Kap123. The deletion of a Cdc42-binding domain (ΔCRIB) increases binding to Kap121. The PAK I binding to karyopherins is separable from its kinase function.
Conclusion: p21-activated kinases could regulate the in vivo availability of β-karyopherin(s). Such a regulation involves the sequestering of karyopherins and increasing the requirement for these proteins expression.