Endoplasmic reticulum is an organelle within a cell. It is divided into smooth and rough endoplasmic reticulum. Smooth endoplasmic reticulum is involved in synthesis and transportation of of lipids while rough endoplasmic reticulum contains ribosomes which are responsible for synthesis of proteins, therefore rough ER is involved in transportation of proteins. A few proteins are synthesized in the mitochondria and the plastids. Therefore, proteins synthesized elsewhere other than the rough ER are the Mitochondrial proteins
