A cisterna (: cisternae) is a flattened membrane vesicle found in the endoplasmic reticulum and Golgi apparatus. Cisternae are an integral part of the packaging and modification processes of proteins occurring in the Golgi.
Function
Proteins begin on the cis side of the Golgi (the side facing the ER) and exit on the trans side (the side facing the plasma membrane). The structure, composition, and function of each of the cisternae may be different inside the Golgi stack. These different variations of Golgi cisternae are categorized into three groups; cis Golgi network, medial, and trans Golgi network. Post-translational modifications such as glycosylation, phosphorylation and cleavage occur in the Golgi and as proteins travel through it, they go through the cisternae, which allows functional ion channels to be created due to these modifications. Each class of cisternae contains various enzymes used in protein modifications. N-linked glycosylation involves the attachment of oligosaccharides to the nitrogen atom of asparagine residues in proteins. These oligosaccharides are composed of various sugar units, including N-acetylglucosamine (GlcNAc), mannose (Man), galactose (Gal), and N-acetylneuraminate (NANA, also known as sialic acid). These glycosylated structures are integral for proper protein function, influencing cellular interactions, protein trafficking, and immune recognition.
N-linked glycosylation begins in the rough endoplasmic reticulum (ER), where a precursor oligosaccharide is synthesized on a lipid carrier called dolichol. The precursor consists of a core structure made up of two N-acetylglucosamine (GlcNAc) residues, nine mannose (Man) residues, and three glucose (Glc) residues.
Once the glycosylated protein enters the ER, further processing of the oligosaccharide occurs. Three specific enzymes play key roles in this early stage of glycosylation. First, glucosidase I removes one glucose residue from the oligosaccharide. Then, glucosidase II removes two more glucose residues, leaving behind a core oligosaccharide attached to the protein. Finally, a mannosidase enzyme removes one mannose residue.