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Description

C-type lectin domain family 1 member B (UniProt Q9P126, also known as C-type lectin-like receptor 2, CLEC-2) is encoded by the CLEC1B (also known as CLEC2, UNQ721/PRO1384) gene (Gene ID 51266) in human. CLEC-2 is a type II single-transmembrane (a.a. 34-54) protein with a large extracellular (a.a. 55-229) region that contains a C-type lectin domain (a.a. 109-217) and a short N-terminal cytoplasmic tail (a.a. 1-33) with a single hem-immunoreceptor tyrosine-based activation motif (hemITAM, a.a. 7-10). CLEC-2 expression is restricted to platelets, where it mediates platelets activation via its hemITAM motif, leading to proteolytic cleavage of two other platelet ITAM receptors, glycoprotein (GP)VI and Fc RIIa, but not of CLEC-2 itself. CLEC-2 functions as the receptor for the type I transmembrane GP podoplanin that is widely expressed outside of the vasculature, including lymphatic endothelial cells, type 1 lung alveolar cells, lymph node stromal cells, the choroid plexus epithelium, inflammatory macrophages, as well as a subset of activated T-helper (Th)17 cells. Patients with rheumatoid arthritis show increased plasma levels of CLEC-2-positive microparticles. These microparticles are derived from activated platelets and are negative for GPVI, while microparticles in healthy donors are predominantly derived from megakaryocytes and are positive for both CLEC-2 and GPVI. Platelet-specific deletion of CLEC-2 or one of its downstream signaling proteins, Syk, SLP-76, or PLC 2, leads to a number of developmental problems in mice, including blood-lymphatic mixing in midgestation.