1) an integrin-binding tripeptide, RGD; 2) a large percentage of the amino acids are acidic; 3) several casein kinase II type phosphorylation sites (SSEE); 4) proline-rich regions (PPPP); and 5) very similar intron-exon structures (e.g. - type 0 introns with exon 1 being noncoding, while exon 2 contains leader sequence + first two amino acids of the mature protein). Heavily lined boxes indicate known splice variants
A family of secreted proteins, termed SIBLINGs (for SmallIntegrin-Binding LIgand, N-linked Glycoproteins), contains the integrin-binding tripeptide, Arg-Gly-Asp (RGD), as well as several conserved phosphorylation and N-glycosylation sites [1]. All of the genes cluster on human chromosome 4 and are normally restricted in expression to skeletal tissue. There are currently six known members of the SIBLING family: bone sialoprotein (BSP), osteopontin (OPN), dentin matrix protein (DMP1), dentin sialophosphoprotein (DSPP), matrix extracellular phosphoglycoprotein (MEPE). Enamelin (ENAM) and bone acidic glycoprotein-75 (BAG75) have also been suggested as members, though their location on chromosome 4 does not coincide with the five clustered members.
