Miraculin-like proteins (MLPs) belong to soybean Kunitz super-family and have been characterized from many plant families
like Rutaceae, Solanaceae, Rubiaceae, etc. Many of them possess trypsin inhibitory activity and are involved in plant defense.
MLPs exhibit significant sequence identity (~30–95%) to native miraculin protein, also belonging to Kunitz super-family compared
with a typical Kunitz family member (~30%). The sequence and structure–function comparison of MLPs with that of a classical
Kunitz inhibitor have demonstrated that MLPs have evolved to form a distinct group within Kunitz super-family. Sequence analysis
of new genes along with available MLP sequences in the literature revealed three major groups for these proteins. A significant
feature of Rutaceae MLP type 2 sequences is the presence of phosphorylation motif. Subtle changes are seen in putative reactive
loop residues among different MLPs suggesting altered specificities to specific proteases. In phylogenetic analysis, Rutaceae
MLP type 1 and type 2 proteins clustered together on separate branches, whereas native miraculin along with other MLPs formed
distinct clusters. Site-specific positive Darwinian selection was observed at many sites in both the groups of Rutaceae MLP
sequences with most of the residues undergoing positive selection located in loop regions. The results demonstrate the sequence
and thereby the structure–function divergence of MLPs as a distinct group within soybean Kunitz super-family due to biotic
and abiotic stresses of local environment.
Keywords Rutaceae – Kunitz super-family – Miraculin-like proteins – Sequence analysis – Evolutionary relationships – Positive selection