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Papers In Press, published online ahead of print March 1, 2005
J. Lipid Res., doi:10.1194/jlr.M400374-JLR200

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Journal of Lipid Research, Vol. 46, 412-421, March 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

Biosynthesis and secretion of insect lipoprotein: involvement of furin in cleavage of the apoB homolog, apolipophorin-II/I

Marcel M. W. Smolenaars^*, Marcelle A. M. Kasperaitis^*, Paul E. Richardson, Kees W. Rodenburg^1,* and Dick J. Van der Horst^*

^* Department of Biochemical Physiology, Faculty of Biology and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands
Department of Chemistry, Coastal Carolina University, Conway, SC 29528

¹ To whom correspondence should be addressed. e-mail: k.w.rodenburg{at}bio.uu.nl

The biosynthesis of neutral fat-transporting lipoproteins involves the lipidation of their nonexchangeable apolipoprotein. In contrast to its mammalian homolog apolipoprotein B, however, insect apolipophorin-II/I (apoLp-II/I) is cleaved posttranslationally at a consensus substrate sequence for furin, resulting in the appearance of two apolipoproteins in insect lipoprotein. To characterize the cleavage process, a truncated cDNA encoding the N-terminal 38% of Locusta migratoria apoLp-II/I, including the cleavage site, was expressed in insect Sf9 cells. This resulted in the secretion of correctly processed apoLp-II and truncated apoLp-I. The cleavage could be impaired by the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (decRVKRcmk) as well as by mutagenesis of the consensus substrate sequence for furin, as indicated by the secretion of uncleaved apoLp-II/I-38. Treatment of L. migratoria fat body, the physiological site of lipoprotein biosynthesis, with decRVKRcmk similarly resulted in the secretion of uncleaved apoLp-II/I, which was integrated in lipoprotein particles of buoyant density identical to wild-type high density lipophorin (HDLp).

These results show that apoLp-II/I is posttranslationally cleaved by an insect furin and that biosynthesis and secretion of HDLp can occur independent of this processing step. Structure modeling indicates that the cleavage of apoLp-II/I represents a molecular adaptation in homologous apolipoprotein structures. We propose that cleavage enables specific features of insect lipoproteins, such as low density lipoprotein formation, endocytic recycling, and involvement in coagulation.

Abbreviations: apoB, apolipoprotein B; apoLp, apolipophorin; decRVKRcmk, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone; HDLp, high density lipophorin; LDLp, low density lipophorin; LDLR, low density lipoprotein receptor; LLT, large lipid transfer; MTP, microsomal triglyceride transfer protein; PC, proprotein convertase

Supplementary key words apolipoprotein B • homology modeling • low density lipophorin • insect lipoprotein receptor • lipovitellin • precursor • proprotein convertase • vitellogenin

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