Cloning and characterization of a mouse homologue of the human haematopoietic cell-specific four-transmembrane gene HTm4

Haematopoietic cell-specific transmembrane-4 (HTm4) is a four-transmembrane protein most closely related to CD20 and the beta subunit of the high affi nity receptor for IgE (Fc epsilon RI beta). To date, it has only been descr ibed in humans, where it is expressed in haematopoietic cells of both myelo id and lymphoid lineages. The function of HTm4 is unknown; however, as for CD20 and Fc epsilon RI-beta, it is likely to play a role in signal transduc tion as part of a multi-subunit cell surface receptor complex. In this stud y, we report the cDNA cloning and expression distribution of mouse HTm4. Th e deduced mouse HTm4 protein is of 213 amino acids, and contains four putat ive transmembrane domains. Mouse HTm4 shows 62% overall amino acid identity with human HTm4; the transmembrane regions are highly conserved between bo th species (75% identity), whereas the N- and C-terminal and inter-transmem brane loop regions are more divergent (52%). Interestingly, the N-terminal domain of mouse HTm4 is predicted to be 23 amino acids shorter, and the C-t erminal domain 23 amino acids longer, than that of human HTm4. Northern blo t and reverse transcriptase (RT)-PCR analysis suggest that mouse HTm4 mRNA is expressed at low levels only in spleen, bone marrow and peripheral blood leucocytes. This is the first report of the cloning of HTm4 from a species other than human, and provides important sequence information towards the understanding of the function of this poorly characterized four-transmembra ne molecule.