What is Plant-specific insert?

The plant-specific insert or plant-specific sequence is an independent domain, exclusively found in plants, consisting of approximately 100 residues, found on the C-terminal lobe on some aspartic proteases called phytepsins. The PSI, as an independent entity separate from its parent AP, is homologous to saposin and belongs to the saposin-like protein family.

Although the PSI is grouped along proteins in the SAPLIP family, the PSI does not contain a proper saposin-like domain. This is due to a circular permutation of the N- and C-termini of the PSI, in which the termini are "swapped". This has led to the PSI being termed a "swaposin" although the tertiary structure still remains homologous to saposin and other members of the SAPLIP family.