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Characterisation of the biosynthesis and processing of the neutrophil granule membrane protein CD63 in myeloid cells
1Department of Haematology, Lund University, Lund, Sweden. malin.ageberg@hematologi.lu.se
Insights
The neutrophil granule protein CD63 undergoes significant processing in myeloid cells, taking over 4 hours to mature through the Golgi apparatus. This study reveals its complex biosynthesis and maturation pathway.
Area of Science:
- Cell Biology
- Protein Biochemistry
- Hematology
Background:
- Neutrophil granule membrane protein CD63 is crucial for azurophil granule function.
- Understanding CD63 biosynthesis and processing is key to comprehending myeloid cell development and function.
Purpose of the Study:
- To investigate the biosynthesis and post-translational modification of CD63 in myeloid cell lines.
- To determine the processing timeline and cellular compartments involved in CD63 maturation.
Main Methods:
- Biosynthetic labeling of four myeloid cell lines.
- Analysis of CD63 molecular weight using SDS-PAGE.
- Enzymatic deglycosylation studies (Endoglycosidase H, N-glycosidase F, O-glycanase).
- Treatment with brefeldin A to assess Golgi apparatus function.
Main Results:
- CD63 is synthesized as 32 and 35 kDa precursors, maturing to a 50 kDa form.
- Glycosylation involves N-linked carbohydrates, with a 23 kDa protein core.
- Maturation requires over 4 hours, indicating transport through medial and trans-Golgi.
- O-linked glycosylation was not detected.
- Golgi apparatus integrity is essential for high molecular weight form generation.
Conclusions:
- CD63 exhibits a complex and relatively long N-linked glycosylation and maturation pathway.
- The Golgi apparatus plays a critical role in processing CD63 to its mature, high molecular weight form.
- Differential synthesis and processing of CD63 were observed across different myeloid cell lines.
Abstract:
The biosynthesis and processing of the neutrophil granule membrane protein CD63, present in azurophil granules, was investigated in four myeloid cell lines. The amount of CD63 synthesised differed, so did the amount of protein processed to high molecular weight forms, with the demonstration of a more prominent synthesis of CD63 in K562 cells. Newly synthesised CD63 was initially detected as two precursor forms with molecular weight of 32 and 35 kDa, respectively. These two initial forms were processed further to yield high molecular weight forms of CD63 with a mean molecular weight of 50 kDa. Treatment with endoglycosidase H or N-glycosidase F revealed a protein core, free from asparagine-linked carbohydrates, with a molecular weight of 23 kDa. Newly synthesised CD63 was susceptible to digestion with endoglycosidase H, and the protein was not completely resistant to endoglycosidase H until after 4 h of chase, indicating that transport through the medial and trans-Golgi complex with conversion of high-mannose carbohydrates to complex oligosaccharide side chains had occurred. This finding indicates a relatively long processing time for CD63 compared to that of soluble azurophil granule proteins. By digestion with O-glycanase, the existence of O-linked oligosaccharides on CD63 could not be demonstrated. Biosynthetic labelling of cells in the presence of brefeldin A showed the importance of a functional Golgi apparatus for processing of the protein to its high molecular weight forms.
