Import inhibition of poly(His) containing chloroplast precursor proteins by Ni²⁺ ions

Abstract

The precursor of the small subunit of ribulose-l,5-bisphosphate carboxylase (pSS) and a modified pSS containing a C-terminal hexahistidyl tail (pSS(His)6) were imported into isolated Chlamydomonas chloroplasts with comparable efficiency. In the presence of Ni 2+ ions the import of pSS(His)6 was inhibited and the precursor bound to the envelope remained protease sensitive, while import of pSS was not affected. Addition of an excess of L-histidine suppressed the inhibition demonstrating that the hexahistidyl-Ni 2+ complex was responsible for import inhibition. Inhibition could be observed between about 0.5 and l0 mM Ni ~+, depending on the total protein content in the assay. Import incompetent Ni2+-precursor complexes can be used to study early events in chloroplast protein import.