Normalization of receptor binding of apolipoprotein E2. Evidence for modulation of the binding site conformation.

Apo E3 is the most common form in the population and is regarded as the parent or normal isoform.

“As apo E3 is the most common form in the population (28, 45, 47), it is regarded as the parent (24) or normal isoform (48).”

In six unrelated patients with type III hyperlipoproteinemia and the apo E-2/2 phenotype, their apo E2 was identical to the most common postulated apo E2 variant.

“In each of six unrelated patients with type III hyperlipoproteinemia and the apo E-2/2 phenotype, we showed that their apo E2 was identical to that which has been postulated as the most common apo E2 variant (24).”

The positively charged arginyl residue at position 158 is thought to maintain the correct conformation of the receptor-binding domain.

“The positively charged arginyl residue at position 158 is thought to maintain the correct conformation of the receptor-binding domain (24).”

A cysteine-for-arginine substitution at position 158 is the most common mutation producing the apo E2 isoprotein.

“We thus confirm that a substitution of cysteine for arginine at position 158 is the most common mutation giving rise to the apo E2 isoprotein.”

Secondary structure predictions suggest that position 136 lies in an alpha-helix from residues 130 to 150, which includes the receptor-binding domain from 140 to 150.

“Secondary structure predictions (54,55) suggest that position 136 is part of an a-helix that extends between residues 130 and 150 (13,24), encompassing the receptor-binding domain from 140 to 150 (22).”

A substitution at position 136 could alter receptor binding, as occurs with apo E2 (158 Arg->Cys), which also lies outside the recognized receptor-binding domain.

“In this way, a substitution at position 136 could alter receptor binding, as happens in the case of apo E2 (158 Arg -* Cys) (24), which also lies outside the recognized receptor-binding domain.”

When the cysteine at position 158 of apoE2 is converted to a positively charged lysine analogue by reaction with cysteamine, this apoE2 possesses ~13% of the receptor binding activity of normal apoE3.

“Similarly, when the cysteine at position 158 of apoE2 is converted to a positively charged lysine analogue by reaction with cysteamine, this apoE2 possesses ~13% of the receptor binding activity of normal apoE3 (25).”

The mutations of apoE3(112 Cys→Arg, 142 Arg→Cys) and apoE1-Harrisburg(146 Lys→Glu) occur within a putative α-helix (residues 131-150).

“In contrast, the mutations of both apoE3(112 Cys→Arg, 142 Arg→Cys) (10) and apoE1-Harrisburg(146 Lys→Glu) (11) occur within a putative α-helix (residues 131-150) (25), which is rich in basic amino acids that are thought to provide direct ionic interaction with the acidic residues within the ligand binding domain of the LDL receptor (31).”

The cysteamine-treated apoE2 22-kDa thrombolytic fragment possesses full receptor binding activity.

“More importantly, the cysteamine-treated apoE2 22-kDa thrombolytic fragment possesses full receptor binding activity (25).”

The carboxyl-terminal domain of apoE can modulate the receptor binding function of the amino-terminal domain.

“Even though the amino- and carboxyl-terminal structural domains of apoE appear to be independently folded (12, 29), evidence suggests that the carboxyl-terminal domain can modulate the receptor binding function of the amino-terminal domain (25).”

The putative α-helix (residues 131-150) is rich in basic amino acids that are thought to provide direct ionic interaction with the acidic residues within the ligand binding domain of the LDL receptor.

“In contrast, the mutations of both apoE3(112 Cys→Arg, 142 Arg→Cys) (10) and apoE1-Harrisburg(146 Lys→Glu) (11) occur within a putative α-helix (residues 131-150) (25), which is rich in basic amino acids that are thought to provide direct ionic interaction with the acidic residues within the ligand binding domain of the LDL receptor (31).”

The apoE2 22-kDa amino-terminal thrombolytic fragment displays a 12-fold increase in binding to the LDL receptor.

“However, the apoE2 22-kDa amino-terminal thrombolytic fragment displays a 12-fold increase in binding to the LDL receptor (25).”