DMP1 and DSPP: evidence for duplication and convergent evolution of two SIBLING proteins.

Since first being proposed as a tandem gene family in 2001, the relatedness of the 5 SIBLING proteins (BSP, DMP1, DSPP, MEPE, and SPP1/OPN) has predominantly depended on arguments involving shared intron/exon properties as well as conserved protein biochemical properties (e.g. unstructured and acidic) and specific peptide motifs (e.g. phosphorylation and integrin-binding RGD). This report discusses the evidence that an ancient DMP1 gene underwent a simple duplication in the common ancestor of mammals and reptiles and then separately evolved into DSPP-like paralogs in the 2 classes. Genomic sequence analyses show that different copies of the original DMP1 duplication process were selected by mammalian and reptilian (anole lizard) classes to acquire genetically different but biochemically similar phosphoserine-rich repeat domains by convergent evolution. Mammals, for example, expanded phosphoserine motifs encoded exclusively using motifs containing AGC/T serine codons while the reptile line's repeats also used TCN-encoding serine codons. A similar analysis of the origins of the other 4 SIBLINGs will require even more detailed analysis as genome sequences of various fish and amphibia become available.