The Signal Peptide Anchors Apolipoprotein M in Plasma Lipoproteins and Prevents Rapid Clearance of Apolipoprotein M from Plasma

Christina Christoffersen,Josefin Ahnström,Olof Axler,Erik Ilsø Christensen,Björn Dahlbäck,Lars Bo Nielsen

doi:10.1074/jbc.m800695200

Christina Christoffersen, Josefin Ahnström + Show 4 more

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https://doi.org/10.1074/jbc.m800695200

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Abstract

Lipoproteins consist of lipids solubilized by apolipoproteins. The lipid-binding structural motifs of apolipoproteins include amphipathic alpha-helixes and beta-sheets. Plasma apolipoprotein (apo) M lacks an external amphipathic motif but, nevertheless, is exclusively associated with lipoproteins (mainly high density lipoprotein). Uniquely, however, apoM is secreted to plasma without cleavage of its hydrophobic NH(2)-terminal signal peptide. To test whether the signal peptide serves as a lipoprotein anchor for apoM in plasma, we generated mice expressing a mutated apoM(Q22A) cDNA in the liver (apoM(Q22A)-Tg mice (transgenic mice)) and compared them with mice expressing wild-type human apoM (apoM-Tg mice). The substitution of the amino acid glutamine 22 with alanine in apoM(Q22A) results in secretion of human apoM without a signal peptide. The human apoM mRNA level in liver and the amount of human apoM protein secretion from hepatocytes were similar in apoM-Tg and apoM(Q22A)-Tg mice. Nevertheless, human apoM was not detectable in plasma of apoM(Q22A)-Tg mice, whereas it was easily measured in the apoM-Tg mice. To examine the plasma metabolism, recombinant apoM lacking the signal peptide was produced in Escherichia coli and injected into wild-type mice. The apoM without signal peptide did not associate with lipoproteins and was rapidly cleared in the kidney. Accordingly, ligation of the kidney arteries in apoM(Q22A)-Tg mice resulted in rapid accumulation of human apoM in plasma. The data suggest that hydrophobic signal peptide sequences, if preserved upon secretion, can anchor plasma proteins in lipoproteins. In the case of apoM, this mechanism prevents rapid loss by filtration in the kidney.

Highlights

The structural motifs conferring the lipid-binding capacity of apolipoproteins has been most intensively studied for apoB in low density lipoprotein [4] and apoA-I in HDL [5, 6]
Like apoM, PON-1 and haptoglobin-related protein (HRP) are mainly found in a subfraction of HDL ( not the same HDL subfraction as that with apoM [9]), and in vitro studies suggest that the signal peptide in PON-1 can mediate its association with lipoproteins [16]
Expression of ApoMQ22A in Transgenic Mice—To examine the impact of the signal peptide sequences on the metabolism of apoM, we used a mutated human apoMQ22A cDNA driven by albumin enhancer-promoter sequences to make Tg mice

Summary

Introduction

The structural motifs conferring the lipid-binding capacity of apolipoproteins has been most intensively studied for apoB in low density lipoprotein [4] and apoA-I in HDL [5, 6]. Like apoM, PON-1 and HRP are mainly found in a subfraction of HDL ( not the same HDL subfraction as that with apoM [9]), and in vitro studies suggest that the signal peptide in PON-1 can mediate its association with lipoproteins [16]. Based on these findings and the highly hydrophobic nature of the signal peptide, we hypothesized that the signal peptide serves as an anchor for apoM in plasma lipoprotein particles. The mice expressing apoMQ22A were compared with transgenic mice expressing wild-type human apoM

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