The LAT story: a tale of cooperativity, coordination, and choreography

Abstract

The adapter molecule LAT is a nucleating site for multiprotein signaling complexes that are vital for the function and differentiation of T cells. Extensive investigation of LAT in multiple experimental systems has led to an integrated understanding of the formation, composition, regulation, dynamic movement, and function of LAT-nucleated signaling complexes. This review discusses interactions of signaling molecules that bind directly or indirectly to LAT and the role of cooperativity in stabilizing LAT-nucleated signaling complexes. In addition, it focuses on how imaging studies visualize signaling assemblies as signaling clusters and demonstrate their dynamic nature and cellular fate. Finally, this review explores the function of LAT based on the interpretation of mouse models using various LAT mutants.

Figure 1.

LAT in TCR signal transduction. (A) Human LAT is a 233 amino-acid type III transmembrane protein with four extracellular amino acids, a single transmembrane region, and a cytosolic region that undergoes multiple posttranslational modifications. Modifications include palmitoylation at cysteine residues C26 and C29 and possible ubiquitylation at lysine residues K52 and K204 (Ubiquitin [Protein Data Bank ID 1UBQ] is represented as a ribbon diagram generated with the program PyMOL). Nine tyrosine residues are conserved between human and mouse LAT sequences, five of which have been shown to undergo phosphorylation (Y127, Y132, Y171, Y191, and Y226). Threonine 155 is also phosphorylated by Erk. (B) Ligation of the TCR induces tyrosine phosphorylation of numerous adapter and effector proteins leading to the activation of multiple signaling pathways important for gene transcription, cytoskeletal reorganization, and cell adhesion. LAT is central to this process by nucleating multiprotein signaling complexes that are important for enzyme activation and signal propagation.

Figure 2.

LAT complexes. Multiple LAT molecules are oligomerized by complexes of 2:1 Grb2:Sos1 or Grb2:Cbl in vitro and in cells. The clustering of LAT complexes is thought to be crucial for signal transduction and possibly for the control of TCR sensitivity to strong or weak stimuli.

Figure 3.

LAT signaling complexes and microclusters. A confocal image of the surface of a Jurkat E6.1 T cell dropped onto a TCR stimulatory coverslip shows LAT clusters, which colocalize with the LAT-binding proteins Grb2 and Gads. Grb2 (upper left) is in blue, LAT (upper middle) is in green and Gads (upper right) is in red. LAT signaling complexes (represented in the cartoon) form a pronounced network of heterogeneous and dynamic microclusters on the plasma membrane (see merge).

Figure 4.

Lymphoproliferative disease in LAT Y136F KI mice. (A) Spleens from 10-week-old wild type C57BL/6 and LAT Y136F KI mice are shown. The ruler depicts centimeters. (B–E) Lymphocyte infiltration into lung. H&E-stained sections of lung from wild type C57BL/6 (B,C) and LAT Y136F KI mice (D,E) are shown. Images were photographed using a 2X (B,D) or 10X (C,E) objective.