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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 intermediate. Enzyme resides in cis-Golgi ensuring early tagging before further trimming. In trans-Golgi network, uncovering enzyme N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase encoded by NAGPA hydrolyzes masking GlcNAc exposing M6P monoester. Only then two P-type lectin receptors 300 kDa cation-independent and 46 kDa cation-dependent bind slightly acidic pH packaging cargo into AP1 and GGA clathrin vesicles to late endosomes where acid releases cargo for lysosomal delivery. Failure cis-phosphorylation causes mucolipidosis II I-cell disease with secreted hydrolases and lysosomal storage phenotype. ER never adds M6P, so phosphorylation definitive marker.

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. Mannosidase II removes terminal alpha1,3 and alpha1,6 mannoses producing GlcNAcMan3GlcNAc2 core. GlcNAc transferase II adds second GlcNAc to alpha1,6 arm creating conserved biantennary GlcNAc2Man3GlcNAc2 platform for further elaboration. Subsequent steps include core fucosylation by FUT8, galactosylation by B4GalT1 and capping with sialic acid by ST3Gal and ST6Gal sialyltransferases in trans-Golgi finalizing complex glycans. Strict order ensures fidelity; reversing mannosidase II before GlcNAc transferase I blocks progression because enzyme specificity strictly depends on prior GlcNAc addition essential for hybrid formation and branched structures.

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 addition licenses subsequent mannosidase II removal and GlcNAc transferase II building biantennary structures and future fucosylation. Without enzyme cells cannot synthesize hybrid or complex glycans and accumulate Man5 as in Lec1 CHO mutants and human CDG-IIa with dysmorphism, neurologic deficits, growth retardation, immunodeficiency, coagulopathy, liver dysfunction. Fucosyl, galactosyl and sialyltransferases operate downstream in medial to trans compartments after initiation critical for glycoprotein maturation and signaling integrity throughout tissues and development.

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

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