Molecular SIBLINGs in Health and Disease

Small Integrin Binding LIgand N-linked Glycoproteins
Family members include:
    bone sialoprotein (BSP), osteopontin (OPN), dentin matrix protein-1 (DMP-1), dentin
    secreted phosphoprotein (DSPP),  and matrix extracellular phosphoglycoprotein (MEPE)

Is this a real family?
    Evidence against:
          no strong homologies at the amino acid level
          overall chemical properties are different
               BSP, DMP1, DSPP and OPN are all acidic while MEPE is strongly basic
               Within acidic members, BSP is glutamic acid-rich, OPN is aspartic acid-rich, while DMP1 is a mix of the two
     Evidence for:
          clustering of genes within a single chromosome
          similarity of their intron-exon boundaries
          biochemical similarities of their corresponding exons
               Exon 1 is always a noncoding exon.
               Exon 2 always contains the start codon, the leader sequence and the codons for the first two amino acids of the mature proteins.
               Exon 3 usually contains a casein kinase II phosphorylation site (SSEE.
               Exon 4 is usually relatively proline-rich (PPPP).
               Exon 5 usually contains another casein kinase II phosphorylation site
               The last one or two exons encode the vast majority of the protein and always contain the integrin-binding tripeptide, RGD
          proteins are flexible in solution
          common binding partners
          common biological effects