Chemokine (C motif) ligand (XCL1) is a small cytokine belonging to the C chemokine family that is also known as lymphotactin. Chemokines are known for their function in inflammatory and immunological responses. This family C chemokines differs in structure and function from most chemokines. There are only two chemokines in this family and what separated them from other chemokines is that they only have two cysteines; one N-terminal cysteine and one cysteine downstream. These both are called Lymphotactin, alpha and beta form, and claim special characteristics only found between the two. Lymphotactins can go through a reversible conformational change which changes its binding shifts.
XCL1's gene is found on the long arm of chromosome 1, located on cytogenetic band q24.2 as seen in the infobox. The encoding gene is made of 6,017 DNA bases to encode for the protein XCL1. This gene contains three exons and two introns as well as several transcription initiation sites. This gene encodes for the 114-amino acid protein called XCL1 which is similar to other chemokines except that it lacks the first and third cysteine characteristics. This means that XCL1 only contains one cysteine creating a disulfide bond instead of two or three like the other chemokines.
The genetic differences between XCL1 and XCL2 are very small. Both proteins are from the same family containing the C motif structure containing one disulfide bond and have almost identical tertiary structures. These C chemokines also have the same flanking regions, meaning regions of the gene including the promoter and other places of protein binging that do not contribute to the RNA transcripted gene. Gene mapping of this chemokine family shows similarities in their intron and exon locations with only one distinct difference. XCL1 has only one difference in its first intron that encodes for a large ribosomal subunit called L7a. In XCL2 have of the region encoding for L7a is cut off. The only other genetic difference between the two mature proteins is the different amino acid in positions 7 and 8. This amino acid difference may account for some biological differences. Some difficulties with comparing these two chemokines is that XCL2 has never been observed in a mouse.
One thing that sets XCL1 apart from other cytokines is its structure. While most chemokines have two disulfide bonds that connect the N-terminus to the core of the structure, XCL1 only has one. This simple difference in disulfide bonds changes the overall tertiary structure of XCL1 from other chemokines. There are two parts of the lymphotactin protein, structures Ltn10 and Ltn40, that folds into each other, which make it biologically active. This conformational change alters the binding structures on the chemokine. This understanding of the interfolding provides more of a basis to understanding to the lymphotactinkinetics.
Most of the secreted XCL1 comes from a specific kind of DC that is involved in antigen cross-presentation. This means that XCL1 is involved in the activation of cytotoxic T cells by a DC. XCL1 can also be secreted by NK cells along with other chemokines in the beginning of infection. This has been associated with the T helper cell type 1 defense. This secretion has also been onserved to facilitate the NK cells to communicate with DC containing XCR1 on their surface. In the same way, secretion of XCL1 encourages Cytotoxic T cells to also communicate with DC containing SCR1.
The pair of XCL1 and XCR1 are known to be involved in cross-presentation, antigen uptake, and induction of innate as well as adaptive cytotoxic immunity.
XCL1 is also known to increate T cells in joints that are effected with rheumatoid arthritis. They are also expressed on RA synovial lymphocytes.
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^ abYoshida T, Imai T, Takagi S, Nishimura M, Ishikawa I, Yaoi T, Yoshie O (October 1996). "Structure and expression of two highly related genes encoding SCM-1/human lymphotactin". FEBS Letters. 395 (1): 82–8. doi:10.1016/0014-5793(96)01004-6. PMID8849694.
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