Insilico functional domain characterisation and phylogenetic analysis of alpha 2 macroglobulin related molecule (α2m_so) from spongia officinalis

A high molecular weight (182 kDa) serum protein, cardiac isoform of α 2 macroglobulin, functions as a protease inhibitor, minimizing cellular damage due to inflammation during cardiac hypertrophy. Based on the biological functions, it was speculated that this protein could have its origin from a primitive metazoan like sponge in response to immune and environmental challenges. This uncertainty prompted us to seek for an alpha 2 macroglobulin related protein in the poriferan, marine sponge, Spongia officinalis. To our surprise, RT PCR for the sponge total RNA with primers for 182 kDa rat serum protein produced a 738 bp amplicon with partial regions homologous to amino terminal (N region) and carboxy terminal (Receptor Binding Region), lacking other functional domains of rat 182kDa cDNA. We cloned and expressed the sponge cDNA for alpha 2 macroglobulin related molecule (α2M_SO) in E.coli and the synthesis of full length protein (~25 kDa) was confirmed with SDS-PAGE analysis. Computational analysis was performed to understand the structural and evolutionary context of the sponge protein. The deduced amino acid sequence contained two signature domains of alpha 2 macroglobulin protein. The three dimensional models predicted by homology modeling for α2M_SO confirmed its role in immunity as a protease inhibitor. Further phylogenetic studies suggested that the evolution of cardiac isoform of α 2 macroglobulin 182 kDa serum protein may have been forced to structural selection that results in genetic modifications leading to gradually highly complex cardiac hypertrophy specific 182kDa serum protein of enhanced stability and diversified functions in higher metazoans. This study thus revealed the existence of an alpha 2 macroglobulin related molecule (α2M_SO) in sponge and traces back to the origin of cardiac isoform of α 2 macroglobulin 182 kDa serum protein.