To identify novel components of the TCR
signaling pathway, a large-scale retroviral-based functional screen was performed using
CD69 expression as a marker for
T cell activation. In addition to known regulators, two truncated forms of p21-activated
kinase 2 (
PAK2), PAK2DeltaL(1-224) and PAK2DeltaS(1-113), both lacking the
kinase domain, were isolated in the
T cell screen. The
PAK2 truncation, PAK2DeltaL, blocked Ag receptor-induced
NFAT activation and TCR-mediated
calcium flux in Jurkat
T cells. However, it had minimal effect on
PMA/ionomycin-induced
CD69 up-regulation in
Jurkat cells, on anti-IgM-mediated
CD69 up-regulation in
B cells, or on the migratory responses of resting
T cells to
chemoattractants. We show that
PAK2 kinase activity is increased in response to TCR stimulation. Furthermore, a full-length kinase-inactive form of
PAK2 blocked both TCR-induced
CD69 up-regulation and
NFAT activity in
Jurkat cells, demonstrating that
kinase activity is required for
PAK2 function downstream of the TCR. We also generated a GFP-fused
PAK2 truncation lacking the
Cdc42/Rac interactive binding region domain, GFP-PAK2(83-149). We show that this construct binds directly to the
kinase domain of
PAK2 and inhibits anti-TCR-stimulated
T cell activation. Finally, we demonstrate that, in primary
T cells, dominant-negative
PAK2 prevented anti-CD3/CD28-induced IL-2 production, and TCR-induced
CD40 ligand expression, both key functions of activated
T cells. Taken together, these results suggest a novel role for
PAK2 as a positive regulator of
T cell activation.
