PAK1-LC8 (Dynein Light Chain) Interaction Required for Nuclear Import of PAK1.

PAK1 is an oncogenic Ser/Thr kinase which is activated by several cytoplasmic
proteins such as GTPases (Rac and CDC42), SH3 adaptor proteins (PIX and
NCK), and Tyr-kinase (ETK). This kinase is required for the growth of more
than 70% of human cancers, including breast and prostate cancers, as well
as formidable brain tumors such as glioma and NF (neurofibromatosis) tumors.

Recently John Williams' group at Thomas Jefferson University in Philadelphia
found that Dynein Light Chain (LC8) also binds residues 212-222 of PAK1
which is adjacent to the essential NLS (nuclear localization site, residues
243-245), and their interaction leads to the dimerization of PAK1 through
NLS and is required for the EGF-induced nuclear import of the cytoplasmic
PAK1 in MCF-7, an estrogen-dependent breast cancer cell line. Since PAK1
phosphorylates the nuclear estrogen receptor (ER) at Ser 305 for the ER
activation which leads to the estrogen-dependent malignant growth of this
cell line, LC8 appears to contribute to the oncogenicity of PAK1 by nuclear
import.


PLoS One. 2009 Jun 26;4(6):e6025

Interaction with LC8 is required for Pak1 nuclear import and is indispensable
for zebrafish development.

Lightcap CM, Kari G, Arias-Romero LE, Chernoff J, Rodeck U, Williams, John
C.

Department of Biochemistry and Molecular Biology, Thomas Jefferson University,
Philadelphia, PA, USA.

Pak1 is a serine/threonine kinase implicated in regulation of cell motility
and survival and in malignant transformation of mammary epithelial cells.
In addition, the dynein light chain, LC8, has been described to cooperate
with Pak1 in malignant transformation of breast cancer cells. Pak1 itself
may aid breast cancer development by phosphorylating nuclear proteins, including
estrogen receptor alpha. Recently, we showed that the LC8 binding site on
Pak1 is adjacent to the nuclear localization sequence (NLS) required for
Pak1 nuclear import.

Here, we demonstrate that the LC8-Pak1 interaction is necessary for epidermal
growth factor (EGF)-induced nuclear import of Pak1 in MCF-7 cells, and that
this event is contingent upon LC8-mediated Pak1 dimerization. In contrast,
Pak2, which lacks an LC8 binding site but contains a nuclear localization
sequence identical to that in Pak1, remains cytoplasmic upon EGF stimulation
of MCF-7 cells.

Furthermore, we show that severe developmental defects in zebrafish embryos
caused by morpholino injections targeting Pak are partially rescued by co-injection
of wild-type human Pak1, but not by co-injection of mutant Pak1 mRNA disrupting
either the LC8 binding or the NLS site.

Collectively, these results suggest that LC8 facilitates nuclear import
of Pak1 and that this function is indispensable during vertebrate development.