Structural differences between wild-type and fish eye disease mutant of lecithin:cholesterol acyltransferase

A retractable lid in lecithin:cholesterol acyltransferase provides a structural mechanism for activation by apolipoprotein A-I

Lecithin:cholesterol acyltransferase (LCAT) plays a key role in reverse cholesterol transport by transferring an acyl group from phosphatidylcholine to cholesterol, promoting the maturation of high-density lipoproteins (HDL) from discoidal to spherical particles. LCAT is activated through an unknown mechanism by apolipoprotein A-I (apoA-I) and other mimetic peptides that form a belt around HDL. Here, we report the crystal structure of LCAT with an extended lid that blocks access to the active site, consistent with an inactive conformation. Residues Thr-123 and Phe-382 in the catalytic domain form a latch-like interaction with hydrophobic residues in the lid. Because these residues are mutated in genetic disease, lid displacement was hypothesized to be an important feature of apoA-I activation. Functional studies of site-directed mutants revealed that loss of latch interactions or the entire lid enhanced activity against soluble ester substrates, and hydrogen-deuterium exchange (HDX) mass spectrometry revealed that the LCAT lid is extremely dynamic in solution. Upon addition of a covalent inhibitor that mimics one of the reaction intermediates, there is an overall decrease in HDX in the lid and adjacent regions of the protein, consistent with ordering. These data suggest a model wherein the active site of LCAT is shielded from soluble substrates by a dynamic lid until it interacts with HDL to allow transesterification to proceed.

A novel frameshift mutation of the lecithin:cholesterol acyltransferase (LCAT) gene associated with renal failure in familial LCAT deficiency

BACKGROUND

The lecithin:cholesterol acyltransferase (LCAT) gene is located on the long arm of chromosome 16 and encodes a highly conserved enzyme that catalyzes the formation of most plasma lipoprotein cholesteryl esters. Two autosomal recessive disorders, familial LCAT deficiency (FLD) and fish eye disease, are associated with germline LCAT mutations. Patients with FLD and fish-eye disease frequently present with corneal opacity, anemia and renal failure with proteinuria.

METHODS

We clinically and biochemically characterized a German patient with classical FLD and used molecular genetic analysis to identify a novel homozygous LCAT mutation within codon 178.

RESULTS

The insertion of adenine identified is located in one of the two motifs that resemble sequences found in several lipases, and results in a frameshift with a stop codon at residue 214. Therefore, the mutation alters a large portion of the LCAT enzyme, including both protein regions with putative lipase activity. Clinically, the female patient presented with corneal opacity, mild anemia and a slow deterioration in kidney function that led to a requirement for hemodialysis until she received a renal transplant.

CONCLUSIONS

The present data provide additional insights into the genotype/phenotype correlations of FLD and thus may improve the genetic diagnosis of this interesting inborn error of metabolism.