Osteopontin

'Osteopontin' (OPN), also known as bone /sialoprotein I (BSP-1 or BNSP), early T-lymphocyte activation (ETA-1), secreted phosphoprotein 1 (SPP1), 2ar and Rickettsia resistance (Ric), is a protein that in humans is encoded by the SPP1 gene (secreted phosphoprotein 1). The murine ortholog is Spp1. Osteopontin is a SIBLING (glycoprotein) that was first identified in 1986 in osteoblasts. The prefix osteo- indicates that the protein is expressed in bone, although it is also expressed in other tissues. The suffix -pontin is derived from "pons," the Latin word for bridge, and signifies osteopontin's role as a linking protein. Osteopontin is an extracellular structural protein and therefore an organic component of bone. The gene has 7 exons, spans 5 kilobases in length and in humans it is located on the long arm of chromosome 4 region 22 (4q1322.1). The protein is composed of ~300 amino acids residues and has ~30 carbohydrate residues attached, including 10 sialic acid residues, which are attached to the protein during post-translational modification in the Golgi apparatus. The protein is rich in acidic residues: 30-36% are either aspartic or glutamic acid. OPN is a highly negatively charged, heavily phosphorylated extracellular matrix protein that lacks an extensive secondary structure as an intrinsically disordered protein. It is composed of about 300 amino acids (297 in mouse; 314 in human) and is expressed as a 33-kDa nascent protein; there are also functionally important cleavage sites. OPN can go through posttranslational modifications, which increase its apparent molecular weight to about 44 kDa. The OPN gene is composed of 7 exons, 6 of which containing coding sequence. The first two exons contain the 5' untranslated region (5' UTR). Exons 2, 3, 4, 5, 6, and 7 code for 17, 13, 27, 14, 108 and 134 amino acids, respectively. All intron-exon boundaries are of the phase 0 type, thus alternative exon splicing maintains the reading frame of the OPN gene.