1. An actin-binding protein (haematopoietic-specific protein-1, haematopoietic cell-specific LYN substrate 1, 486 aa) homologous to cortactin and restricted to leucocytes. Becomes phosphorylated in response to ligation of the T-cell receptor and together with WASP, WAVE, and cofilin is involved in triggering the formation of an F-actin scaffold at the immunological synapse and may be involved in signalling through T-cell lck or B-cell lyn tyrosine kinases. Not required in platelets. HS1-binding protein-3 (HS1-BP3, 392 aa) associates with SH3 domains of HS1 and may be part of the signalling complex. Overexpression of mutant HS1BP3 protein in T-cell lines decreases IL-2 production.
2. Tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein theta isoform (YWHAQ, 14-3-3 protein θ, HS1, 245 aa), one of the *14-3-3 family of signalling proteins, strongly expressed in brain and to a lesser extent in lung, kidney, liver, and spleen. Has significant sequence similarity with stratifin. It is reported to be consistently up-regulated in the spinal cord of patients with amyotrophic lateral sclerosis.