TCR Triggering Induces the Formation of Lck–RACK1–Actinin-1 Multiprotein Network Affecting Lck Redistribution

Abstract

The initiation of T-cell signaling is critically dependent on the function of the member of Src family tyrosine kinases, Lck. Upon T-cell antigen receptor (TCR) triggering, Lck kinase activity induces the nucleation of signal-transducing hubs that regulate the formation of complex signaling network and cytoskeletal rearrangement. In addition, the delivery of Lck function requires rapid and targeted membrane redistribution, but the mechanism underpinning this process is largely unknown. To gain insight into this process, we considered previously described proteins that could assist in this process via their capacity to interact with kinases and regulate their intracellular translocations. An adaptor protein, receptor for activated C kinase 1 (RACK1), was chosen as a viable option, and its capacity to bind Lck and aid the process of activation-induced redistribution of Lck was assessed. Our microscopic observation showed that T-cell activation induces a rapid, concomitant, and transient co-redistribution of Lck and RACK1 into the forming immunological synapse. Consistent with this observation, the formation of transient RACK1–Lck complexes were detectable in primary CD4+ T-cells with their maximum levels peaking 10 s after TCR–CD4 co-aggregation. Moreover, RACK1 preferentially binds to a pool of kinase active pY394Lck, which co-purifies with high molecular weight cellular fractions. The formation of RACK1–Lck complexes depends on functional SH2 and SH3 domains of Lck and includes several other signaling and cytoskeletal elements that transiently bind the complex. Notably, the F-actin-crosslinking protein, α-actinin-1, binds to RACK1 only in the presence of kinase active Lck suggesting that the formation of RACK1–pY394Lck–α-actinin-1 complex serves as a signal module coupling actin cytoskeleton bundling with productive TCR/CD4 triggering. In addition, the treatment of CD4+ T-cells with nocodazole, which disrupts the microtubular network, also blocked the formation of RACK1–Lck complexes. Importantly, activation-induced Lck redistribution was diminished in primary CD4+ T-cells by an adenoviral-mediated knockdown of RACK1. These results demonstrate that in T cells, RACK1, as an essential component of the multiprotein complex which upon TCR engagement, links the binding of kinase active Lck to elements of the cytoskeletal network and affects the subcellular redistribution of Lck.

CMIP is a negative regulator of T cell signaling

“… 36 Finally, (4) actin cytoskeleton rearrangement structurally regulates LR dynamics. 37 We have previously shown that CMIP interacts physically with Fyn and filamin A, so it may interfere with both LR and cytoskeleton dynamics. 13 , 38 Altogether, these data suggest that the negative regulatory role of CMIP is complex and affects early events in T cell signaling.…”

NFATc1 controls the cytotoxicity of CD8+ T cells

“…6 ). The expression of Actn1 gene encoding α-actinin 1 contributes to T cell activation by forming multiprotein complexes with Lck and Rack1 that couple actin skeleton with TCR triggering, and it was shown to participate in T cell migration 50 , 51 .…”

Proteomic Analysis of Lipid Rafts from RBL-2H3 Mast Cells

“…RACK1 adopts a highly conserved seven-bladed β-propeller structure that serves as binding sites for multiple partners [106,107]. Recently, RACK1 was identified as a crucial component of a multiprotein complex formed in T-cell lipid rafts upon TCR (T-cell antigen receptor) activation [108]. Despite the significant role that RACK1 plays in shuttling and anchoring proteins and its involvement in immunoregulatory responses [107,108,109], RACK1 has not previously been described in MCs.…”