The answer is the endoplasmic reticulum, and I will tell you how is it possible.
The protein is translated in ribosomes from mRNAs, then some of these proteins are transported to the endoplasmic reticulum by the ribosomes (that's why there's rough endoplasmic reticulum, it is due to the ribosomes in the membrane of the ER).
These proteins undergo post-translational modifications by adding and/or removing parts to/from the protein, these modifications represent signals for the golgi apparatus (which is the next step for protein maturation) to know where this protein is supposed to go (inside, outside, in a membrane or in an organelle).
