The primary role of endoplasmic reticulum (ER) in protein synthesis is to facilitate the synthesis and folding of proteins. The rough ER, dotted with ribosomes, is specifically involved in translating and processing proteins destined for secretion or for use in cell membranes. These proteins often undergo modifications such as glycosylation. The smooth ER plays a role in lipid synthesis and detoxification. Together, the ER collaborates with the golgi apparatus to ensure proper protein maturation and transport.