Most microtubules in a cell consist of a simple tube, a singlet microtubule, built from 13 protofilaments. 

TLS Online TPP Program

#Id: 7950

#Unit 2. Cellular Organization

Prior to transfer to a nascent chain in the lumen of the  ER, the oligosaccharide precursor is assembled on amembrane- attached anchor called dolichol phosphate, a longchain polyisoprenoid lipid

TLS Online TPP Program

#Id: 7951

#Unit 2. Cellular Organization

First sugar residue is attached to dolichol by a high-energy pyrophosphate linkage & the other sugars are added by glycosidic bonds

TLS Online TPP Program

#Id: 7952

#Unit 2. Cellular Organization

Two N- acetylglucosamine (GlcNAc) and five mannose residues are added one at a time to a dolichol phosphate on the cytosolic face of the ER membrane

TLS Online TPP Program

#Id: 7953

#Unit 2. Cellular Organization

Tunicamycin

blocks the first enzyme in this pathway, inhibits the synthesis of all N-linked oligosaccharides in cells.

TLS Online TPP Program

#Id: 7954

#Unit 2. Cellular Organization

The precursor oligosaccharide is transferred from a

dolichol lipid anchor to the asparagine as an intact unit in a reaction catalyzed by a transmembrane oligosaccharyl transferase enzyme complex.

TLS Online TPP Program

#Id: 7955

#Unit 2. Cellular Organization

Only asparagine residues in the tripeptide sequences Asn-X-Ser and Asn-X-Thr (where X is any amino acid except proline) are substrates for oligosaccharyl transferase.