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#Golgi

3 public questions tagged with this topic.

Protein Sorting and Trafficking

In which Golgi compartment does phosphorylation of lysosomal enzymes occur?

Soluble acid hydrolases acquire mannose-6-phosphate marker for lysosomal delivery through selective two-step modification restricted to Golgi apparatus, discovered by Kornfeld laboratory. Newly synthesized hydrolases enter ER, receive high-mannose N-glycans, fold with calnexin cycle, then travel to cis-Golgi network. There UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase alpha2 beta2 gamma2 hexamer encoded by GNPTAB and GNPTG recognizes conformational patch on hydrolases and transfers GlcNAc-1-phosphate to C6 hydroxyl of select mannose residues forming phosphodiester interm

Ref: Lodish et al., MCB, Chapter 14 Figure 14-21 Formation of M6P residues in cis-Golgi by phosphotransferase.

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Protein Sorting and Trafficking

What is the correct sequence of glycosylation steps in the Golgi?

Conversion of high-mannose precursors to complex N-glycans requires spatial separation of Golgi enzymes across cis, medial and trans cisternae enforcing substrate channeling and ordered maturation preventing futile cycles and ectopic branching errors. Early cis-Golgi contains alpha-mannosidase I trimming Man9GlcNAc2 to Man5GlcNAc2 removing alpha1,2 mannoses requiring calcium cofactor and interaction with cargo receptors. Next medial-Golgi resident GlcNAc transferase I adds GlcNAc to alpha1,3 mannose branch generating GlcNAcMan5GlcNAc2 now high-affinity substrate for medial mannosidase II. Mann

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 15: Ordered Golgi N-glycan processing sequence.

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Protein Sorting and Trafficking

Which enzyme in the Golgi is responsible for adding the first GlcNAc during N-linked glycosylation?

N-glycan processing begins in endoplasmic reticulum with en bloc transfer of Glc3Man9GlcNAc2 and trimming to Man8GlcNAc2, but creation of hybrid and complex structures occurs in Golgi. Upon entry to cis-Golgi, alpha-mannosidase I removes four alpha1,2 mannoses yielding compact Man5GlcNAc2, obligate substrate for committed step. In medial-Golgi, GlcNAc transferase I product of MGAT1 is type II membrane protein with short cytosolic tail and large luminal catalytic domain transferring N-acetylglucosamine from UDP-GlcNAc to C2 position of alpha1,3 mannose arm generating GlcNAcMan5GlcNAc2. This add

Ref: Essentials of Glycobiology, NCBI Bookshelf, Chapter 9 N-Glycans: GlcNAc-TI in medial-Golgi initiates complex N-glycan formation.

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