Complete amino-acid sequence and carbohydrate content of the naturally occurring glucosylceramide activator protein (A1 activator) absent from a new human Gaucher disease variant.

Two naturally occurring non-enzymic glucosylceramide activator proteins (A1a and A1b activator) shown previously to be immunochemically not detectable in a new variant of human Gaucher disease (glucosylceramide lipidosis) without glucosylceramidase deficiency, were characterized by amino-acid sequence and carbohydrate content. The complete amino-acid sequence of the A1a activator was determined. The protein consists of 80 amino-acid residues including three disulfide bridges lacking arginine and tryptophan. The molecular mass is 8.95 kDa. About 20% of the polypeptide chain are shorter by two amino-acid residues at the N-terminal end. The A1b activator was characterized by the amino-acid compositions of all tryptic peptides and of the entire protein; sequencing was performed of the regions 1-34 and 42-56. Identical results were obtained for the polypeptide chains of both A1 activators. This suggests that they do not differ in their primary structures which is in agreement with the immunochemical results. The difference between A1a and A1b activator is due to the carbohydrate part. The total amount of 49% carbohydrate in A1a and 76.7% in A1b consists mainly of hexoses. Both chains contain two moles of N-acetylglucosamine per mole protein bound to asparagine in position 22. A comparison of the primary structure of the A1 activator with the sulfatide activator sequence revealed an interesting similarity, especially of the cysteine residues and the carbohydrate-binding asparagine. Sequence homology was also found between a part of the A1 activator sequence and the hemagglutinin neuraminidase of influenza virus as well as to a hypothetical glycoprotein of the Epstein-Barr virus. The comparison with human lysosomal glucosylcerebrosidase showed no sequence similarity.