Predominant cell surface sialoprotein of leukocytes which regulates multiple T-cell functions, including T-cell activation, proliferation, differentiation, trafficking and migration. Positively regulates T-cell trafficking to lymph-nodes via its association with ERM proteins (EZR, RDX and MSN) (PubMed:17638845, PubMed:21289089, PubMed:11728336). Negatively regulates Th2 cell differentiation and predisposes the differentiation of T-cells towards a Th1 lineage commitment (PubMed:18490738). Promotes the expression of IFN-gamma by T-cells during T-cell receptor (TCR) activation of naive cells and induces the expression of IFN-gamma by CD4(+) T-cells and to a lesser extent by CD8(+) T-cells. Plays a role in preparing T-cells for cytokine sensing and differentiation into effector cells by inducing the expression of cytokine receptors IFNGR and IL4R, promoting IFNGR and IL4R signaling and by mediating the clustering of IFNGR with TCR (By similarity). Acts as a major E-selectin ligand responsible for Th17 cell rolling on activated vasculature and recruitment during inflammation. Mediates Th17 cells, but not Th1 cells, adhesion to E-selectin (PubMed:26700769). Acts as a T-cell counter-receptor for SIGLEC1 (PubMed:11238599). Protects cells from apoptotic signals, promoting cell survival. Interacts with SIGLEC1. Interacts with isoform 2 of HIPK2 (PubMed:11078605). Interacts with CTNNB1 (By similarity). Interacts with RDX (via FERM domain) (PubMed:18614175). Interacts with EZR (PubMed:21289089). Interacts with MSN (By similarity) (PubMed:18614175, PubMed:21289089). Localizes to the uropodium and microvilli via its interaction with ERM proteins (EZR, RDX and MSN). The sumoylated form localizes to the PML body. Cell surface of thymocytes, T-lymphocytes, neutrophils, plasma cells and myelomas. Phosphorylation at Ser-347 is regulated by chemokines, requires its association with ERM proteins (EZR, RDX and MSN) and is essential for its function in the regulation of T-cell trafficking to lymph nodes. Has a high content of sialic acid and O-linked carbohydrate structures. Cleavage by CTSG releases its extracellular domain and triggers its intramembrane proteolysis by gamma-secretase releasing the CD43 cytoplasmic tail chain (CD43-ct) which translocates to the nucleus. Sumoylated.