Two-step processing of in vivo synthesized rice lectin.

The synthesis and processing of rice lectin was followed in vivo in developing rice embryos. Using labelling and pulse-chase labelling experiments, the sequence of events in the synthesis and post-translational modifications of this protein could be determined. The primary lectin product observed in vivo is a high molecular weight precursor (28 K), which is post-translationally converted to a 23 K lectin protein, and in a further step cleaved into two smaller 12 K and 10 K polypeptides. The first step of the processing of the rice lectin is a rather slow process (the precursor has a half-life of about 3 h) and resembles the so-called vectorial processing of cytoplasmically made organellar proteins. The second modification consists of a (slow) proteolytic cleavage of the basic lectin subunit into two smaller polypeptides and resembles somewhat the cleavage of some legume (storage) proteins in their protein bodies.