Secretion of a soluble class I molecule encoded by the Q10 gene of the C57BL/10 mouse

The DNA sequence of the Q10 genes appears to be highly conserved amongst strains of mice and has only been found to be transcribed in the liver. An examination of the nucleotide sequence of the exon that normally encodes the transmembrane domain of class I molecules suggested that the Q10 gene encodes a secreted protein. We have established this by showing that L cells transformed with an expression vector containing the Q10 gene secrete a class I molecule which was identified with an antiserum raised against a peptide predicted by the Q10 transmembrane exon. Both the L cell-derived Q10 molecule and a class I protein immunoprecipitated from serum with this anti-peptide antiserum have mol. wts. of approximately 38 000; the Q10 molecule secreted by L cells is heterogeneous in mol. wt. This heterogeneity was drastically reduced after endoglycosidase F treatment, suggesting that Q10 molecules secreted into the serum by the liver may be glycosylated differently from those secreted by L cells. Endoglycosidase F treatment of both the L cell and serum forms of the soluble molecule yielded two products with mol. wts. of approximately 32 000 and 35 000; this is consistent with the observation that the predicted Q10 protein sequence has two potential glycosylation sites. In contrast to previous published results, the Q10 molecule reacted with rabbit anti-H-2 antisera which is consistent with its greater than 80% homology to the classical transplantation antigens.