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

Abstract

Summary Haematopoietic cell-specific transmembrane-4 (HTm 4 ) is a four-transmembrane protein most closely related to CD20 and the beta subunit of the high affinity receptor for IgE (FcεRIβ). To date, it has only been described in humans, where it is expressed in haematopoietic cells of both myeloid and lymphoid lineages. The function of HTm 4 is unknown; however, as for CD20 and FcεRI-β, it is likely to play a role in signal transduction as part of a multi-subunit cell surface receptor complex. In this study, we report the cDNA cloning and expression distribution of mouse HTm 4 . The deduced mouse HTm 4 protein is of 213 amino acids, and contains four putative transmembrane domains. Mouse HTm 4 shows 62% overall amino acid identity with human HTm 4 ; the transmembrane regions are highly conserved between both species (75% identity), whereas the N-and C-terminal and inter-transmembrane loop regions are more divergent (52%). Interestingly, the N-terminal domain of mouse HTm 4 is predicted to be 23 amino acids shorter, and the C-terminal domain 23 amino acids longer, than that of human HTm 4 . Northern blot and reverse transcriptase (RT)-PCR analysis suggest that mouse HTm 4 mRNA is expressed at low levels only in spleen, bone marrow and peripheral blood leucocytes. This is the first report of the cloning of HTm 4 from a species other than human, and provides important sequence information towards the understanding of the function of this poorly characterized four-transmembrane molecule.

The cell cycle associated protein, HTm4, is expressed in differentiating cellsof the hematopoietic and central nervous system in mice

“…Immunoprecipitation studies and western blotting confirm a similar biochemical association between murine HTm4 and KAP protein in a murine leukocyte cell line, P815, suggesting that murine HTm4 plays a similar role in mouse and human leukocytes. Although we have already detected HTm4 expression in primary human mast cells, basophils, eosinophils (Adra et al 1994, Nakajima et al 2004, and also in murine P815, another group did not detect HTm4 mRNA in P815 by PCR (Hulett et al 2001). This could be due to a very low copy number of mRNA that nevertheless corresponds to a very high level or protein.…”

“…Murine HTm4 is highly similar to human HTm4 at the DNA and amino acid level, and both proteins display a similar predicted four-transmembrane structure (Figure 1). Comparison to two reported HTm4 cDNA sequences (Hulett et al 2001;) revealed two amino acid sense variants (Ser162Ala and Ser211Thr). CLUSTAL alignment analysis and chromosomal grouping places murine HTm4, like human HTm4, in the MS4 protein superfamily (Hulett et al 2001, Ishibashi et al 2001.…”

“…Comparison to two reported HTm4 cDNA sequences (Hulett et al 2001;) revealed two amino acid sense variants (Ser162Ala and Ser211Thr). CLUSTAL alignment analysis and chromosomal grouping places murine HTm4, like human HTm4, in the MS4 protein superfamily (Hulett et al 2001, Ishibashi et al 2001. A similar genomic structure between the two genes is also identified with both genes containing a six-intron and seven-extron arrangement.…”

“…To date, few members of this family are well understood, but a diverse functionality is beginning to emerge. These functions include roles as cell surface signaling receptors and intracellular adapter proteins (Hulett et al 2001, Ishibashi et al 2001, Barnhill et al 2004. We demonstrated that HTm4 is an adapter molecule that regulates the function of the KAP phosphatase, a key regulator of cell cycle progression (Donato et al 2002).…”

Binding of HTm4 to Cyclin-dependent Kinase (Cdk)-associated Phosphatase (KAP)·Cdk2·Cyclin A Complex Enhances the Phosphatase Activity of KAP, Dissociates Cyclin A, and Facilitates KAP Dephosphorylation of Cdk2

“…However, a diverse functionality is beginning to emerge. These functions include roles such as cell surface signaling receptors and intracellular adapter proteins (1)(2)(3)(4)(5)22).…”