A single amino acid substitution in a common african allele of the CD4 molecule ablates binding of the monoclonal antibody, OKT4

Abstract

The CD4 molecule is a relatively non-polymorphic 55 kDa glycoprotein expressed on a subset of T lymphocytes. A common African allele of CD4 has been identified by non-reactivity with the monoclonal antibody, OKT4. The genetic basis for the OKT4 − polymorphism of CD4 is unknown. In the present paper, the structure of the CD4 molecule from an homozygous CD4 OKT4− individual was characterized at the molecular level. The size of the CD4 OKT4− protein and mRNA were indistinguishable from those of the OKT4 + allele. The polymerase chain reaction (PCR) was used to map the structure of CD4 OKT4− cDNAs by amplifying overlapping DNA segments and to obtain partial nucleotide sequence after asymmetric amplification. PCR was then used to clone CD4 OKT4− cDNAs spanning the coding region of the entire, mature CD4 protein by amplification of two overlapping segments followed by PCR recombination. The nucleotide sequence of CD4 OKT4− cDNA clones revealed a G → A transition at bp 867 encoding an arginine → tryptophan substitution at amino acid 240 relative to CD4 OKT4+. Expression of a CD4 OKT4−cDNA containing only this transition, confirmed that the arginine → tryptophan substitution at amino acid 240 ablates the binding of the mAb OKT4. A positively charged amino acid residue at this position is found in chimpanzee, rhesus macaque, mouse and rat CD4 suggesting that this mutation may confer unique functional properties to the CD4 OKT4− protein.