Effect of Phosphatase Inhibitor on Protein Import Into Chloroplast

Abstract

Chloroplasts are double membraned organelles which require a lot of proteins to maintain their structure and to perform different functions. The majority of these proteins are encoded in the nucleus and synthesized in the cytoplasm as higher molecular weight precursors carrying an NH₂-terminal domain called a transit sequence. The transit sequence is necessary and sufficient to direct a polypeptide into the chloroplast. In many cases, the cytosolic factors are required to keep the precursor in an unfolded conformation. The import process starts with binding of precursor proteins to the envelope membrane via the interaction of the transit sequence with proteinaceous receptors at the surface of the outer membrane. Then the precursor inserts across the outer and inner two translocation apparatus to arrive in the stroma, where the transit sequence is removed by a stromal processing peptidase. The import process requires the hydrolysis of both ATP and GTP. Recently some investigators have provided evidence that some chloroplast precursors are stay outside of the chloroplast for a long time. These observations indicate that a regulation mechanism exists for protein import into chloroplasts. In the present study, we provide evidence that the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSS) from Chlamydomonas reinhardtii is phosphorylated during its synthesis in wheat germ extract. The inhibition of this precursor import into chloroplasts by phosphatase inhibitor suggests that protein import into chloroplast may be regulated by phosphorylation.