Structural Comparison of the Precursor and the Mature Form of Napin, the 2S Storage Protein in Brassica napus

The 2S storage protein napin from Brassica napus var. L. is synthesised as a precursor protein at the endoplasmic reticulum and transported along a gradient of decreasing pH to the vacuole, where two propeptides are removed to produce mature napin. The structures of pronapin expressed in insect cells and mature napin from rape seed were characterised. Limited proteolysis with several endoproteases cleaved primarily in the propeptides, suggesting that the propeptides are exposed to the exterior of the protein. Immunological comparison in parallel with circular dichroic spectrometry, both at neutral and acid pH, indicated that the propeptides had only a minor influence on the conformation of the regions of the molecule that correspond to mature napin.