No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of the rapid advances in the medical sciences, the Publisher recommends that independent verification of diagnoses and drug dosages should be made. Special regulations for readers in the USA-This publication has been registered with the Copyright Clearance Center Inc. (CCC) Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.

We have studied the influence of phospholipase C treatment of intact purified chloroplast on the translocation of a plastid destined precursor protein. Under standard import conditions, i.e. in the light in the presence or 2 mM ATP translocation was completely abolished but binding was observed at slightly elevated levels. An experimental regime which allowed binding but not import of the precursor protein, i.e. in the dark in the presence of 10 μM ATP, demonstrated that translocation intermediates, normally detected at this stage, were missing in phospholipase treated chloroplasts. The precursor was completely sensitive to protease treatment, indicating that the transfer of the precursor from the receptor to the import apparatus was blocked by phospholipase treatment.

Chloroplasts are highly structured plant specific organelles. They possess three discrete membrane systems that differ in composition and function, i.e. the outer/inner envelope and the thylakoid membranes. In addition three solute spaces can be distinguished, i.e. the space between the envelope membranes, the stroma and the thylakoid lumen. While most of the chloroplastic proteins, which are synthesized as precursors in the cytosol, seem to follow a common route of translocation into the organelle, proteins of the outer envelope, which is in direct contact with the cytosol, are inserted (imported) by a very different and distinct mechanism. A typical polypeptide destined for the inside of the organelle, possesses a cleavable target sequence, retains a loosely folded conformation with the help of molecular chaperones, is recognized by proteinaceous receptors on the organellar surface, requires low concentration (pM) ATP for binding but high concentrations (mM) for complete translocation through the membranes. Outer envelope polypeptides (OEP) studied so far, do not possess a cleavable target sequence, do not require protease sensitive receptors on the organellar surface and do not require ATP for either binding or insertion into the outer envelope.