Different isoforms of an apoprotein (apolipophorin III) associate with lipoproteins in Locusta migratoria

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Abstract
Insects transport lipid for flight in the form of diacylglycerol‐rich low‐density lipoproteins (low‐density lipophorin, LDLp), which in the hemolymph are produced from high‐density lipophorin (HDLp) by reversible association with several molecules of an apolipoprotein, apolipophorin III (apoLp‐III, Mr∼ 18000–20000) during lipid loading. Two isoforms of apoLp‐III (a and b) were purified both from adult Locusta migratoria migratorioides hemolymph and LDLp, which have identical apparent Mr but differ in amino acid composition, NH2‐terminal amino acid sequence, and isoelectric points (5.35 ± 0.01 for apoLp‐IIIa, 5.10 ± 0.01 for apoLp‐IIIb). The NH2‐terminal sequence of apoLp‐IIIb is identical to the primary structure of apoLp‐III deduced from cloned cDNA [Kanost et al. (1988) J. Biol. Chem. 263, 10568–10573], whereas the NH2‐terminal sequence of apoLp‐IIIa is identical to that of apoLp‐IIIb but preceded by Arg‐Pro‐, which is the C‐terminal of the putative signal peptide coded by cDNA upstream from that coding for apoLp‐IIIb. The ratio apoLp‐IIIa apoLp‐IIIb free in hemolymph is identical to that in LDLp (5:9); since 14 molecules of apoLp‐III appear to be bound in one molecule of LDLp, an average of 5 molecules of apoLp‐IIIa and 9 of apoLp‐IIIb are involved in formation of each LDLp particle. In vivo studies using 35S‐labeled apoLp‐IIIa and b demonstrate that each of the isoforms can associate with HDLp to produce LDLp reversibly; in an in vitro system, production of LDLp containing exclusively apoLp‐IIIa or apoLp‐IIIb demonstrates independent participation of each isoform in LDLp formation.

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