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Evidence that a Chloroplast Surface Protein Is Associated with a Specific Binding Site for the Precursor to the Small Subunit of Ribulose-1,5-Bisphosphate Carboxylase.

Most chloroplast proteins are encoded by nuclear genes and synthesized in the cytoplasm as higher molecular weight precursors. These precursors are imported posttranslationally into the chloroplasts, where they are proteolytically processed, and sorted to their proper locations. The first step of this import process is thought to be the binding of precursors to putative receptors on the outer envelope membrane of chloroplasts. We have investigated the interaction of the precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase with its putative receptor by using a heterobifunctional, photoactivatable cross-linker. The resulting cross-linked conjugate has a molecular weight of 86,000, and is present on the surface of chloroplasts as determined by its sensitivity to digestion with protease. Control experiments demonstrated that the label in the conjugate is derived from small subunit precursor and that the conjugate is formed only when modified precursor is reacted in the presence of chloroplasts. Based on these results, we postulate that a protein on the surface of chloroplasts is part of the receptor which interacts with the small subunit precursor.