Serine 6 of Lck tyrosine kinase: A critical site for Lck myristoylation, membrane localization, and function in T lymphocytes

Lck is a member of the Src family kinases expressed predominantly in T cells, and plays a pivotal role in TCR-mediated signal transduction, Myristoylation of glysine 2 in the N-terminal Src homology 4 (SH4) domain of Lck is essential for membrane localization and function. In this study, we examined a site within the SH4 domain of Lck regulating myristoylation, membrane localization, and function of Lck, A Lck mutant in which serine 6 (Ser(6)) was substituted by an alanine was almost completely cytosolic in COS-7 cells, and this change of localization was associated with a drastic inhibition of myristoylation in this mutant. To assess the role of Ser(6) of Lck in T cell function, we established stable transfectants expressing various Lck mutants using Lck-negative JCaM1 cells. The Lck mutant of Ser(6) to alanine, most of which did not target to the plasma membrane, was not able to reconstitute TCR-mediated signaling events in JCaM1 cells, as analyzed by tyrosine phosphorylation of intracellular proteins and CD69 expression. These results demonstrate that Ser(6) is a critical factor for I,ck myristoylation, membrane localization, and function in T cells, presumably because the residue is important for N-myristoyl transferase recognition.