Complete amino acid sequence and homologies of human erythrocyte membrane protein band 4.2.

The complete amino acid sequence for human erythrocyte band 4.2 has been derived from the nucleotide sequence of a full-length 2.35-kilobase (kb) cDNA. The 2.35-kb cDNA was isolated from a human reticulocyte cDNA library made in the expression vector lambda gt11. Of the 2348 base pairs (bp), 2073 bp encode 691 amino acids representing 76.9 kDa (the SDS/PAGE molecular mass is 72 kDa). RNA blot analysis of human reticulocyte total RNA gives a message size for band 4.2 of 2.4 kb. The amino acid sequence of band 4.2 has homology with two closely related Ca2(+)-dependent cross-linking proteins, guinea pig liver transglutaminase (protein-glutamine gamma-glutamyltransferase; protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) (32% identity in a 446-amino acid overlap) and the a subunit of human coagulation factor XIII (27% identity in a 639-amino acid overlap), a transglutaminase that forms intermolecular gamma-glutamyl-epsilon-lysine bonds between fibrin molecules. The region of greatest identity includes a 49-amino acid stretch of band 4.2, which is 69% and 51% identical with guinea pig liver transglutaminase and the a subunit of factor XIII, respectively, within the regions that contain the active sites of these enzymes. Significantly, within the five contiguous consensus residues of the transglutaminase active site, Gly-Gln-Cys-Trp-Val, band 4.2 has an alanine substituted for cysteine (which is apparently essential for activity). Consistent with this active site substitution, erythrocyte membranes or inside-out vesicles, which contain band 4.2, show no evidence of transglutaminase activity by two types of in vitro assay.