A noncoding ABO variant suggests a trafficking defect in COPI-GOLPH3-mediated glycosyltransferase recycling

We report a mechanism linking an intronic ABO splicing mutation to weak A antigen expression. A two-base deletion in intron 1 (c.28 + 2_28 + 3delTG) disrupts canonical splicing, causing aberrant extension of the cytoplasmic tail of the A-type glycosyltransferase (GTA), a key biological macromolecule responsible for blood group biosynthesis. Integration of long-read sequencing, splicing assays, and molecular dynamics simulations revealed that the mutation spares the catalytic domain but alters tail conformation, disrupting its interaction with the Coat Protein Complex I (COPI) adaptors, Golgi phosphoprotein 3 (GOLPH3) and Golgi phosphoprotein 3-like (GOLPH3L). Consequently, GTA recycling is predicted to be blocked, leading to protein mislocalization and reduced A antigen levels. Mechanistically, by occluding the critical L-X-X-(R/K) motif and perturbing tail electrostatics, the mutation is predicted to abrogate productive engagement of GTA with the COPI-GOLPH3/GOLPH3L retrieval machinery. This work establishes protein mislocalization as a cause of ABO weak phenotypes and provides a framework for understanding how noncoding variants can dysregulate the function of glycosyltransferases through spatial control.