Tenascin X (TNX) is a 450 kDa, 4266 amino acid (aa) extracellular matrix glycoprotein belonging to the tenascin family of adhesion proteins (1, 2). Tenascins are modular proteins containing heptad, EGF‑like and fibronectin type III repeats and a C‑terminal fibrinogen‑like domain (1 ‑ 3). The Tenascin XB2 form, also called XB‑S or XB‑short, is transcribed from a separate promoter, producing a 74 kDa protein that consists of the C‑terminal 673 aa of the full‑length TNX (3 ‑ 5). Of the TNX domains, Tenascin XB2 contains only the last 4 of ~32 fibronectin type III (FnIII) repeats and the C‑terminal fibrinogen‑like globular domain, along with some of the N‑glycosylation and serine phosphorylation sites (3 ‑ 5). Human Tenascin XB2 shares 84% aa sequence identity with corresponding regions of mouse and rat TNX. Tenascin XB2 is expressed mainly in the adrenal gland, with minor amounts of mRNA detected in the small intestine, lung and spleen (3, 6). It is also found in some human cell lines, such as MCF7, H1299 and HeLa, and can be induced by hypoxia in MCF‑7 cells (6, 7). In contrast, full‑length TNX is prominent in muscle and connective tissue, and deletion or mutation of TNX is one cause of human joint hypermobility syndromes such as the Ehlers‑Danlos syndrome (1 ‑ 3, 6, 8, 9). Tenascin XB2 lacks an RGD integrin binding site between FnIII 10 and 11, and decorin/collagen‑binding sites that include the EGF‑like repeats, but retains the collagen‑binding fibrinogen‑like domain and could conceivably interfere with collagen crosslinking and fibril formation (4, 9). However, a 75 kDa plasma form that overlaps with the sequence of Tenascin XB2 binds tropoelastin but not collagens (10). At least a portion of Tenascin XB2 is expressed in the cytoplasm and is co‑localized with the mitotic motor kinesin, Eg5, during interphase and mitosis (6, 7).
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