IMPORT INHIBITION OF POLY(HIS) CONTAINING CHLOROPLAST PRECURSOR PROTEINS BY NI2+ IONS

The precursor of the small subunit of ribulose-1,5-bisphosphate carbox ylase (pSS) and a modified pSS containing a C-terminal hexahistidyl ta il (pSS(His)(6)) were imported into isolated Chlamydomonas chloroplast s with comparable efficiency. In the presence of Ni2+ ions the import of pss(His)(6) was inhibited and the precursor bound to the envelope r emained protease sensitive, while import of pSS was not affected, Addi tion of an excess of L-histidine suppressed the inhibition demonstrati ng that the hexahistidyl-Ni2+ complex was responsible for import inhib ition. Inhibition could be observed between about 0.5 and 10 mM Ni2+, depending on the total protein content in the assay. Import incompeten t Ni2+-precursor complexes can be used to study early events in chloro plast protein import. (C) 1997 Federation of European Biochemical Soci eties.