Crystal structure of plant aspartic proteinase prophytepsin: inactivation and vacuolar targeting

We determined at 2.3 Angstrom resolution the crystal structure of prophytep sin, a zymogen of a barley vacuolar aspartic proteinase. In addition to the classical pepsin-like bilobal main body of phytepsin, we also traced most of the propeptide, as well as an independent plant-specific domain, never b efore described in structural terms. The structure revealed that, in additi on to the propeptide, 13 N-terminal residues of the mature phytepsin are es sential for inactivation of the enzyme. Comparison of the plant-specific do main with NK-lysin indicates that these two saposin-like structures are clo sely related, suggesting that all saposins and saposin-like domains share a common topology. Structural analysis of prophytepsin led to the identifica tion of a putative membrane receptor-binding site involved in Golgi-mediate d transport to vacuoles.