Biochemical analysis of a human epithelial surface antigen: differential cell expression and processing

Area of Science:

• Biochemistry
• Cell Biology
• Immunology

Background:

• Epithelial surface antigen (ESA) is a glycoprotein primarily found on human epithelial cells.
• Previous research indicated ESA presents as two chains (38 kDa and 32 kDa) under reducing conditions and one (38 kDa) under non-reducing conditions.
• The antibody MH99 detects ESA, and its relationship to the colon cancer antigen detected by antibody 17-1A is unclear.

Purpose of the Study:

• To elucidate the molecular composition and processing of Epithelial surface antigen (ESA).
• To investigate the mechanism generating cell-type specific variations in ESA expression.
• To determine the relationship between ESA detected by antibodies MH99 and 17-1A.

Main Methods:

• Pulse-chase experiments to track ESA synthesis and processing.
• Peptide mapping to analyze protein structure.
• Sequential immunoprecipitation to compare antibody binding.
• Analysis of ESA under reducing and non-reducing conditions using SDS-PAGE.

Main Results:

• The 38-kDa non-reduced ESA band comprises a 38-kDa single chain and a disulfide-linked dimer of 32-kDa and 6-kDa chains.
• ESA is synthesized as a 34-kDa precursor, glycosylated to 38 kDa, and can be processed into 32-kDa and 6-kDa forms.
• Cell lines exhibit varying ratios of 38-kDa and 32-kDa ESA, with conversion occurring in mixed cell cultures.
• Antibodies MH99 and 17-1A detect closely related or identical antigens.

Conclusions:

• ESA undergoes post-translational modification and proteolytic cleavage, generating cell-type specific variants.
• A novel mechanism for cell surface glycoprotein diversity involves limited proteolysis of ESA.
• The ESA antigen is likely identical or closely related to the colon cancer antigen recognized by antibody 17-1A.