Final answer:
The incorrect statement about peroxisomal proteins is that they have to be unfolded before import. They can keep their tertiary structure while being imported into peroxisomes, differing from the ER import process.
Step-by-step explanation:
The statement that is NOT true regarding peroxisomal proteins is: A. Peroxisomal proteins have to be unfolded before import. Peroxisomal protein import does not require the proteins to be unfolded; instead, they are often translated in the cytosol and retain their tertiary structure as they are imported into the peroxisome.
Proteins that are destined for peroxisomes usually have a specific signal sequence that is recognized by peroxisomal import machinery. This is different from the co-translational import of proteins into the endoplasmic reticulum, where they are inserted into the ER lumen or membrane as they are being synthesized by ribosomes on the rough ER. Once synthesized, peroxisomal proteins do not enter the ER but are directly targeted to the peroxisomes.
Most proteins, including those for peroxisomes, are indeed encoded by nuclear genes (C), and not all peroxisomal proteins are synthesized by ribosomes attached to the rough ER (B). They reach the peroxisome after their synthesis is completed (D), which is a true statement for peroxisomal proteins.