Molecular cloning of mouse erythrocyte protein 4.2: a membrane protein with strong homology with the transglutaminase supergene family.

We report the molecular cloning and characterization of mouse erythrocyte protein 4.2 (P4.2). Mouse erythrocyte P4.2 is a 691-amino-acid protein with a predicted MW of 77 kDa. Northern blot analysis detected a 2.2-kb transcript in mouse reticulocytes, compared with a 2.4- to 2.5-kb transcript in human reticulocytes, which is consistent with the absence of the 30-amino-acid splicing insert in mouse erythrocyte P4.2 that is found in the human protein (isoform I). Like the human erythrocyte P4.2, mouse erythrocyte P4.2 contains regions strikingly homologous with the transglutaminase (TGase) proteins although it too most likely lacks TGase crosslinking activity. Mouse P4.2 is on average 73% identical with human erythrocyte P4.2, although regional variations exist, with greatest conservation in the regions of the molecule that contain the TGase active site, the TGase calcium-binding site, and a band 3 binding site. Hydropathy analysis reveals a protein containing a series of hydrophobic domains, similar to the situation for human P4.2 and consistent with its tight binding to the membrane, although the mouse P4.2 is missing both the strongly hydrophilic region and adjacent highly charged region that are present in the human protein, suggesting that the two proteins could differ in their physical characteristics, binding associations, or functional properties. The availability of the complete mouse erythrocyte P4.2 cDNA should help in the design of P4.2-deficient animal models (for example, ribozyme or homologous recombinant "knockout" models) that should accelerate the understanding of P4.2 function in both erythroid and non-erythroid cells.