Structure, intracellular targeting, and function of barley aspartic proteinase

Proteolytic enzymes belonging to the aspartic proteinase class are important e.g. in the dairy industry (animal and fungal chymosins and, occasionally, plant derived rennets) and in medicine (pepsin, renin, cathepsins D and E). Therefore, animal and fungal aspartic proteinases have been under extensive research for a long time. Most animal aspartic proteinases are known to have well defined physiological functions. These proteinases activate or inactivate other enzymes or are involved in their degradation.

Athough purified from different plant species and tissues, plant aspartic proteinases are relatively poorly characterized. Their physiological function is still obscure although some roles have been suggested, such as digestion of storage proteins at the onset of germination, processing of 2S albumin precursors in Arabidopsis, as well as digestion of pathogenesis-related proteins in tomato leaves.

Barley aspartic proteinase (HvAP = Hordeum vulgare aspartic proteinase) was originally purified from barley grains, where it exists as two enzymatically active heterodimeric forms of 32+16 kDa and 29+11 kDa, putatively derived by sequential processing of the same proenzyme. We have studied the primary structure and enzymatic properties of HvAP, and its 3-D structure has been computer modelled. The enzyme resembles closely mammalian cathepsin D and yeast proteinase A both in its structural and hydrolytic features as well as in its vacuolar (lysosomal) localization. However, HvAP differs from its mammalian and microbial counterparts by containing a 104 residues plant specific insert at its C-terminal lobe; part of this insert area is removed during processing of the enzyme.

We have also studied the expression of HvAP in different barley tissues and its possible physiological role in the processing of the C-terminal vacuolar targeting signal of barley lectin. Currently we are interested in finding out the vacuolar targeting signal of HvAP itself.

Selected publications