LAT (Linker for Activation of T-cells) is a protein involved in signaling through the T-cell receptor (TCR). The invention provides a LAT protein including mutations at ubiquitylation sites that result in an increase in stability of LAT in stimulated and unstimulated cells, and enhanced signaling through the TCR. The invention further provides use for a LAT protein including mutations at ubiquitylation sites for therapeutic and laboratory methods.