How are proteins normally transported into a plant cell’s chloroplast, if they were originally synthesized on ribosomes present in the cytosol (not on the rough endoplasmic reticulum)? A. they are packaged into vesicles with v-SNARES for fusion with their target organelles B. they are unfolded in the translocation channel, then refolded by chaperones C. they are secreted by the Golgi, captured by receptors, and absorbed by endosomes D. they must pass through the nuclear envelope by means of nuclear pores E. they can pass freely through large pores in the inner chloroplast membrane
The correct answer and explanation is:
The correct answer is B. they are unfolded in the translocation channel, then refolded by chaperones.
Proteins that are synthesized in the cytosol, particularly those destined for the chloroplast, do not go through the rough endoplasmic reticulum or Golgi apparatus. Instead, they are directly transported into the chloroplast via a specialized process involving the outer and inner chloroplast membranes. These proteins usually contain a specific targeting sequence, known as the “chloroplast transit peptide,” which guides them to the chloroplast.
The transport mechanism occurs through translocation channels called Tic and Toc complexes. The Toc complex, located in the outer membrane, first recognizes the transit peptide of the protein and facilitates its passage into the intermembrane space. The protein is then transferred to the Tic complex in the inner membrane, which actively translocates the protein into the stroma of the chloroplast.
Once inside, the protein is usually unfolded during translocation because the size of the pores in the membranes is too small for folded proteins to pass through. Chaperones in the stroma, such as Hsp70, bind to the unfolded protein and help it refold into its functional three-dimensional shape. The chloroplast transit peptide is often cleaved off during or after translocation, leaving the mature protein ready for its role in the chloroplast.
This process is essential for maintaining proper function in chloroplasts, as many critical proteins involved in processes such as photosynthesis, protein synthesis, and metabolism are encoded by nuclear DNA and synthesized in the cytosol. They must be imported into the chloroplast to carry out their functions.
Thus, option B correctly describes the process by which proteins are unfolded during transport and then refolded inside the chloroplast.