Asn54-linked glycan is critical for functional folding of intercellular adhesion molecule-5

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a type I membrane glycoprotein involved in dendritic polarization and filopodia formation.
• While ICAM-5's N-glycan structures are known, their functional roles remain unclear.

Purpose of the Study:

• To elucidate the function of N-glycans on ICAM-5 by mutating each potential N-glycosylation site.
• To investigate the impact of these mutations on ICAM-5 trafficking, cell surface expression, and filopodia formation in neuronal cells.

Main Methods:

• Generation of fifteen ICAM-5 gene constructs with mutated N-glycosylation sites.
• Transfection of constructs into Neuro-2a (N2a) cells and observation of filopodia-like protrusions.
• Immunofluorescence staining, cell surface biotinylation, and enzymatic digestion (Endo H, PNGase) to assess protein localization and glycosylation status.
• Analysis of protein complex formation, disulfide bond formation, and ER-associated degradation pathways.

Main Results:

• Mutation of the N54 site (N54Q mutant) abolished ICAM-5's ability to induce filopodia-like protrusions.
• N54Q ICAM-5 was retained in the ER, failed to reach the cell surface, and primarily contained high-mannose-type glycans.
• The N54Q mutant exhibited impaired disulfide bond formation and failed to form functional protein complexes.
• Stable N54Q transformants showed retarded cell growth but no significant ER stress, indicating degradation via the ER-associated pathway.

Conclusions:

• The Asn(54)-linked glycan is essential for the normal trafficking, cell surface expression, and functional activity of ICAM-5.
• This critical glycan is necessary for proper protein folding, disulfide bond formation, and complex assembly.
• The Asn(54)-linked glycan's role in ICAM-5 function is independent of ER-associated degradation pathways.