Small guanine nucleotide–binding proteins (G proteins) activated by antigen recognition stimulate at least three different mitogen-activated protein (MAP) kinases, which in turn activate distinct transcription factors. G proteins are involved in diverse activation responses in different cell types. Two major members of this family activated downstream of the TCR are RAS and RAC. Each activates a different transcription factor, and together they mediate many cellular responses of T cells.
• The RAS pathway is triggered in T cells after TCR ligation, leading to the activation of extracellular receptor–activated kinase (ERK), a prominent member of the MAP kinase family, and eventually to the activation of downstream transcription factors (Fig. 1). RAS is loosely attached to the plasma membrane through covalently bound lipids. In its inactive form, the guanine nucleotide–binding site of RAS is occupied by guanosine diphosphate (GDP). When the bound GDP is replaced by guanosine triphosphate (GTP), RAS undergoes a conformational change and can then recruit or activate various cellular enzymes, the most important of which is RAF. Activation of RAS by GDP/GTP exchange is seen in response to the engagement of many types of receptors in many cell populations, including the TCR complex in T cells. Mutated RAS proteins that are constitutively active (i.e., they constantly assume the GTP-bound conformation) contribute to the neoplastic transformation of many cell types. Nonmutated RAS proteins are active GTPases that convert the GTP bound to RAS into GDP, thus returning RAS to its normal, inactive state.
Fig1. The RAS–mitogen-activated protein (MAP) kinase pathway in T-cell activation. ZAP70, activated by antigen recognition, phosphorylates membrane-associated adaptor proteins (such as linker for activation of T cells [LAT]) that then bind another adaptor, GRB2, which provides a docking site for the guanosine tri phosphate/guanosine diphosphate (GTP/GDP) exchange factor SOS. SOS converts RAS ⋅ GDP to RAS ⋅ GTP. RAS ⋅ GTP activates a cascade of enzymes, which culminates in the activation of the MAP kinase extracellular receptor–activated kinase (ERK). A parallel RAC-dependent pathway generates another active MAP kinase, c-Jun N-terminal kinase (not shown).
The mechanism of RAS activation in T cells involves the adaptor proteins LAT and GRB2. When LAT is phosphorylated by ZAP70 at the site of TCR clustering, it serves as the docking site for the SH2 domain of GRB2. Once attached to LAT, GRB2 recruits a RAS GTP/GDP exchange factor called SOS to the plasma membrane. SOS catalyzes GTP for GDP exchange on RAS. This generates the GTP-bound form of RAS (written as RAS⋅GTP), which then triggers a cascade of kinase activation. RAS⋅GTP directly activates a kinase called RAF, the first kinase in this cascade. RAF then phosphorylates and activates a dual-specificity kinase called MEK1, which in turn phosphorylates the third kinase in the cascade, called ERK, on closely spaced threonine and tyrosine residues. ERK is a MAP kinase, and MEK1 is called a MAP kinase-kinase (a kinase that activates a MAP kinase). The activated ERK translocates to the nucleus and phosphorylates a protein called ELK, and phosphorylated ELK stimulates transcription of FOS, which encodes a component of the activator protein 1 (AP-1) transcription factor.
• In parallel with the activation of RAS through recruitment of GRB2 and SOS, the adaptors phosphorylated by TCR associated kinases also recruit and activate a GTP/GDP exchange protein called VAV that acts on RAC, another small guanine nucleotide–binding protein. The RAC⋅GTP that is generated initiates a parallel MAP kinase cascade, resulting in the activation of a distinct MAP kinase, c-Jun N-terminal kinase (JNK). JNK is sometimes called a stress-activated protein (SAP) kinase because in many cells it is activated by various noxious stimuli. Activated JNK then phosphorylates JUN, the second component of the AP-1 transcription fac tor. A third member of the MAP kinase family, in addition to ERK and JNK, is p38, and it too is activated by RAC⋅GTP and in turn activates various transcription factors. RAC⋅GTP also induces cytoskeletal reorganization and may play a role in the clustering of TCR complexes, coreceptors, and other signaling molecules into the synapse.
The activities of ERK and JNK are eventually shut off by the action of dual-specificity protein tyrosine/threonine phosphatases. These phosphatases are induced or activated by ERK and JNK themselves, providing a negative feedback mechanism to terminate T-cell activation.
