The Q4 class I gene has been shown to participate
in gene conversion events within the mouse major
histocompatibility complex. Its complete genomic nucleotide
sequence has been determined. The 5' half of Q4
resembles H-2 genes more strongly than other Q genes.
Its 3' end, in contrast, is Q-like and contains a translational
stop signal in exon 5 which predicts a polypeptide with
an incomplete membrane spanning segment. The presence
of two inverted B1 repeats suggests that part of the Q4
gene may be mobile within the genome. Gene transfer experiments
have shown that the Q4 gene encodes a
ß2-microglobulin associated polypeptide of Mr 41 000. A
similar protein was found in activated mouse spleen cells.
The Q4 polypeptide was found to be secreted both by
spleen cells and by transfected fibroblasts and was not detectable
on the cell surface. Antibody binding and twodimensional
gel electrophoresis indicate that the Q4
molecule is identical to a mouse class I polypeptide, Qb-1,
which has been previously described.