Apolipoprotein A-I domains involved in lecithin-cholesterol acyltransferase activation. Structure:function relationships.

The properties of the mature and pro-forms of recombinant apolipoprotein A-I (apoA-I) were compared with those of apoA-I isolated from human plasma. When the synthesis and secretion of pro-and mature forms of apoA-I from a baculovirus/insect cell expression system were compared in parallel experiments, the amount of the pro-form of apoA-I synthesized and secreted was severalfold higher than that of the mature form of apoA-I. A comparison of the properties of the pro-and mature forms of recombinant apoA-I and human plasma apoA-I showed no difference between all three in their secondary structure, their ability to self-associate, lipid-binding capacity, lecithin: cholesterol acyltransferase activation, and binding to the phospholipid transfer protein. The properties of reconstituted high density lipoprotein (HDL) particles formed from the proteins and their ability to promote cholesterol and phospholipid efflux from human skin fibroblasts were also similar. However, their ability to bind to plasma HDL subfractions differed, because twice as much proapoA-I associated with pre ␤ 1 -HDL and pre ␤ 2 -HDL subfractions compared with both mature recombinant and plasma apoA-I. Correspondingly, the amount of proapoA-I in ␣ -HDL subfractions, especially in ␣ 1 -HDL and ␣ 2 -HDL, was decreased. We conclude that while the propeptide of apoA-I is required for the effective synthesis and secretion of apoA-I, cleavage of this peptide is a requisite for the effective interconversion of HDL subfractions. -Sviridov, D., L. E. Pyle, M. Jauhiainen, C. Ehnholm, and N. H. Fidge. Deletion of the propeptide of apolipoprotein A-I reduces protein expression, but stimulates effective conversion of pre ␤ -high density lipoprotein to ␣ -high density lipoprotein.

Section: Discussionsupporting

confidence: 85%

Section: Discussionmentioning

confidence: 99%

Deletion of the propeptide of apolipoprotein A-I reduces protein expression but stimulates effective conversion of preβ-high density lipoprotein to α-high density lipoprotein

Sviridov

Pyle

Jauhiainen

et al. 2000

“…Recent studies have now clearly established that region 144-186 is the LCAT activator domain of apoA-I (123,(157)(158)(159)(160). After alignment of these helices with other LCAT activating peptides, we have proposed an LCAT activating motif (123) (Fig.…”

Section: A Lcat Activationmentioning

confidence: 91%

Apolipoprotein A-I: structure–function relationships

Frank

Marcel

2000

266

The inverse relationship between high density lipoprotein (HDL) plasma levels and coronary heart disease has been attributed to the role that HDL and its major constituent, apolipoprotein A-I (apoA-I), play in reverse cholesterol transport (RCT). The efficiency of RCT depends on the specific ability of apoA-I to promote cellular cholesterol efflux, bind lipids, activate lecithin:cholesterol acyltransferase (LCAT), and form mature HDL that interact with specific receptors and lipid transfer proteins. From the intensive analysis of apoA-I secondary structure has emerged our current understanding of its different classes of amphipathic ␣ -helices, which control lipid-binding specificity. The main challenge now is to define apoA-I tertiary structure in its lipid-free and lipid-bound forms. Two models are considered for discoidal lipoproteins formed by association of two apoA-I with phospholipids. In the first or picket fence model, each apoA-I wraps around the disc with antiparallel adjacent ␣ -helices and with little intermolecular interactions. In the second or belt model, two antiparallel apoA-I are paired by their C-terminal ␣ -helices, wrap around the lipoprotein, and are stabilized by multiple intermolecular interactions. While recent evidence supports the belt model, other models, including hybrid models, cannot be excluded. ApoA-I ␣ -helices control lipid binding and association with varying levels of lipids. The N-terminal helix 44-65 and the C-terminal helix 210-241 are recognized as important for the initial association with lipids. In the central domain, helix 100-121 and, to a lesser extent, helix 122-143, are also very important for lipid binding and the formation of mature HDL, whereas helices between residues 144 and 186 contribute little. The LCAT activation domain has now been clearly assigned to helix 144-165 with secondary contribution by helix 166-186. The lower lipid binding affinity of the region 144-186 may be important to the activation mechanism allowing displacement of these apoA-I helices by LCAT and presentation of the lipid substrates.No specific sequence has been found that affects diffusional efflux to lipid-bound apoA-I. In contrast, the C-terminal helices, known to be important for lipid binding and maintenance of HDL in circulation, are also involved in the interaction of lipid-free apoA-I with macrophages and specific lipid efflux. While much progress has been made, other aspects of apoA-I structure-function relationships still need to be studied, particularly its lipoprotein topology and its interaction with other enzymes, lipid transfer proteins and receptors important for HDL metabolism. -Frank, P. G., and Y. L. Marcel. Apolipoprotein A-I: structure-function relationships.

“…Activation of LCAT by apoA-I, the major apolipoprotein component of HDL, has been clearly demonstrated and has been shown to depend directly upon the binding of apoA-I to the lipid surface (30,31). The domain extending from 143 to 208 of the mature apoA-I protein seems to play a critical role in the overall LCAT reaction (32). The most important factor, apparently, is the nature of the HDL particle substrate, i.e., the detailed physicochemical characteristics of the particle surface where the interaction of LCAT with substrate takes place (33).…”

Section: Discussionmentioning

confidence: 99%

LCAT facilitates transacylation of 17β-estradiol in the presence of HDL3 subfraction

Höckerstedt

Tikkanen

Jauhiainen

2002

It has been shown that estrogens need to be metabolized to their hydrophobic estrogen ester derivatives to act as antioxidants in lipoproteins. Data suggest that 17 ␤estradiol (E 2 ) becomes esterified in LCAT-induced reactions and the esters are transported from HDL particles to LDL and VLDL particles by a CETP-dependent mechanism. In the present study we have further investigated the regulation of E 2 esterification by LCAT and focused on the importance of HDL structure and composition in the esterification process. Isolated LDL, HDL 2 , HDL 3 , and reconstituted discoidal HDL (rHDL) were incubated with labeled E 2 , with and without purified LCAT, at 37 ؇ C for 24 h. After purification of the lipoprotein fractions, there was a significant peak of radioactivity representing esterified estradiol attached to HDL 3 and rHDL, but HDL 2 and LDL contained only trace amounts of labeled estradiol ester. TLC analysis confirmed that the radioactivity migrated in a position corresponding to that of 17 ␤ -E 2 17-monoester standard. The amount of radioactivity associated with HDL 3 and rHDL representing esterified E 2 was significantly increased by addition of purified LCAT. However, only limited increases of radioactivity were observed in HDL 2 and LDL. In conclusion, HDL subfractions differ in their potential to regulate estradiol esterification by LCAT. -Höckerstedt, A., M. J. Tikkanen, and M. Jauhiainen. LCAT facilitates transacylation of 17 ␤ -estradiol in the presence of HDL 3 subfraction.