Identification of functional, short-lived isoform of linker for activation of T cells (LAT).

Linker for activation of T cells (LAT) is a transmembrane adaptor protein playing a key role in the development, activation and maintenance of peripheral homeostasis of T cells. In this study we identified a functional isoform of LAT. It originates from an intron 6 retention event generating an in-frame splice variant of LAT mRNA denoted as LATi6. Comparison of LATi6 expression in peripheral blood leukocytes of human and several other mammalian species revealed that it varied from being virtually absent in the mouse to being predominant in the cow. Analysis of LAT isoform frequency expressed from minigene splicing reporters carrying loss- or gain-of-function point mutations within intronic polyguanine sequences showed that these elements are critical for controlling the intron 6 removal. The protein product of LATi6 isoform (LATi6) ectopically expressed in LAT-deficient JCam 2.5 cell line localized correctly to subcellular compartments and supported T-cell receptor signaling but differed from the canonical LAT protein by displaying a shorter half-life and mediating an increased interleukin-2 secretion upon prolonged CD3/CD28 crosslinking. Altogether, our data suggest that the appearance of LATi6 isoform is an evolutionary innovation that may contribute to a more efficient proofreading control of effector T-cell response.