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.