Acquired immunity is largely dependent on T cells. Upon the recognition of antigen by T cell, protein phosphorylation cascades downstream of T cell receptor (TCR) are triggered. It was previously shown that cytoskeletal F-actin is involved in TCR signaling. However, how such F-actin is regulated and exerts its function in TCR signaling remains unclear.
In this study, we investigated the role of the actin nucleating and polymerizing protein, formin, in TCR signaling by the using of a formin inhibitor, SMIFH2. We found that while phosphorylation of ZAP-70 remains intact, the phosphorylation of a ZAP-70 substrate, LAT, was strongly suppressed upon SMIFH2 treatment. In addition, we found that phosphorylated ZAP-70 colocalized with phosphorylated LAT on the cell membrane upon TCR stimulation in control cells, suggesting that LAT is phosphorylated by phosphorylated ZAP-70 on the cell membrane. Moreover, we employed superesolution microscopy and found that TCR stimulation-induced F-actin remodeling occurs in the cell cortex and requires formin's activity. These findings together therefore suggest that formin-dependent cortical F-actin remodeling is critical for the phosphorylation of LAT by ZAP-70 on the cell membrane.