[3] The PSI, as an independent entity separate from its parent AP, is homologous to saposin and belongs to the saposin-like protein family (SAPLIP).

[4] Among plants, APs between different species are generally homologous exhibiting high sequence identity whilst maintaining a similar tertiary structure to pepsin.

In the case of Procardosin A, the zymogenic form of Cardosin A (the major AP found in cardoon), the PSI is first removed before activation of the mature enzyme occurs in which the prosegment is cleaved during proteolytic processing.

[3][6][7] Moreover, sequence alignment between NK-lysin and PSI reveals that the relative positions of the disulfide bridges are also conserved, a common trait among SAPLIPS.

[4] Several crystal structures exist, containing the coordinates of the zymogenic parent AP and the SAPLIP domain of the PSI, for cardoon and barley.

Potato PSI overexpressed separately in A. thalina increases resistance to the pathogen Botrytis cinerea, both by its own antifungal activity and by its ability to induce plant defenses.