Synthesis and processing of thionin precursors in developing endosperm from barley (Hordeum vulgare)

Abstract

Thionin is a lysine-rich polypeptide (mol. wt. 5000) which is synthesized in developing barley endosperm from - 8 days to - 30 days after anthesis. Two thionin precursors (THPl and THP2) have been identified using monospecific antibodies (A-TH) prepared against the mature protein. THPl, which is the only polypeptide recognized in vitro by A-TH, is encoded by a 7.5S mRNA obtained from membrane-bound polysomes, and its alkylated derivative has an apparent mol. wt. of 17 800. THP2, which is selected together with mature thionin by A-TH among labelled proteins in vivo, differs from THPl in apparent mol. wt. (17 400 alkylated) and in electrophoretic mobility at pH 3.2. Both THPl and THP2 are competed out of the antigen-antibody complex by purified thionin. The conversión of THP2 into thionin, which has been demonstrated in a pulse-chase experiment in vivo, is a post-translational process. As it has not been possible to detect THPl in vivo it is assumed that it is converted cotranslationally into THP2. Final deposition of thionin as an extrínsic membrane protein, possibly associated with the endoplasmic reticulum, has been tentatively established on the basis of subcellular fractionation experíments