The importance of NFAT in T-cell activation is illustrated by the effects of selective inhibitors of calcineurin called cyclosporin A (CsA) and tacrolimus (also known as FK506). CsA forms a complex with the protein cyclophilin A, and this complex inhibits calcineurin. Tacrolimus binds a different protein, FK-binding protein (FKBP), making a complex that similarly inhibits calcineurin. By inhibiting calcineurin, these drugs prevent the formation of active NFAT. T cells express low levels of calcineurin, so they are more sensitive to inhibition of this pathway than are many other cell types. Both cyclosporin A and tacrolimus thus act as effective immunosuppressants, and are widely used to prevent the rejection of organ transplants (see Chapter 16, Section 16-3).
7-15 Ras activation stimulates the mitogen-activated protein kinase (MAPK) relay and induces expression of the transcription factor AP-1.
A second branch of the PLC-γ signaling module is the activation of the small GTPase Ras. This can occur by various means. The most efficient mechanism for Ras activation in T cells is via the DAG generated by PLC-γ, which diffuses in the plasma membrane and activates a variety of proteins. One of these is the protein RasGRP, which is a guanine-nucleotide exchange factor that specifically activates Ras. RasGRP contains a protein-interaction module called a C1 domain that binds to DAG. This interaction recruits RasGRP to the membrane near active signaling complexes (Fig. 7.19), where it activates Ras by promoting the exchange of GDP for GTP. Ras is also activated in the T-cell receptor signaling pathway by the guanine-exchange factor Sos, which is recruited by the adaptor protein Grb2 (see Sections 7-2 and 7-3), which has itself been recruited by binding to phosphorylated LAT and SLP-76.
Ras is initially inactive. TCR signaling produces DAG, which recruits RasGRP to the membrane where it activates Ras Ras activates Raf, which phosphorylates Mek, which phosphorylates Erk Activated Erk enters the nucleus and activates transcription factors such as Elk-1 inactive Ras RasGRP active Ras DAG Raf transcription factor nucleus cytoplasm Mek Erk
Activated Ras then triggers a three-kinase relay that ends in the activation of a serine/threonine kinase known as a mitogen-activated protein kinase or MAP kinase (MAPK) (see Fig. 7.19). In the case of antigen receptor signaling, the first member of the relay is a MAPK kinase kinase (MAP3K) called Raf. Raf is a serine/threonine kinase that phosphorylates the next member of the series, a MAPK kinase (MAP2K) called MEK1. MEK1 is a dual-specificity protein kinase that phosphorylates a tyrosine and a threonine residue on the last member of the relay, a MAPK which in T cells and B cells is Erk (extracellular signal-related kinase).
Signaling by MAPK cascades is facilitated by specialized scaffold proteins that bind to all three kinases in a particular MAPK relay, thereby accelerating their interactions. The scaffold protein kinase suppressor of Ras (KSR) functions in the Raf/MEK1/Erk pathway. During T-cell receptor signaling, KSR associates with Raf, MEK1, and Erk and localizes itself and its cargo to the membrane. In that location, activated Ras can engage with the Raf bound to KSR and trigger the kinase relay (see Fig. 7.19).
An important function of MAPKs is to phosphorylate and activate transcription factors that can then induce new gene expression. Erk acts indirectly to generate the transcription factor AP-1, which is a heterodimer composed of one monomer each from the Fos and Jun families of transcription factors (Fig. 7.20). Active Erk phosphorylates the transcription factor Elk-1, which