Insect apolipophorin III: interaction of locust apolipophorin III with diacylglycerol.
In the formation of low-density lipophorin (LDLp) by the loading of diacylglycerol onto high-density lipophorin (HDLp) in insect hemolymph, apolipophorin III (apoLp-III) plays an essential role by binding to the increasing surface of the expanding lipoprotein particle. The present data on the surface properties of apoLp-III from Locusta migratoria demonstrate a preferential interaction with diacylglycerol. Injection of apoLp-III underneath a diacylglycerol monolayer results in a rapid interaction with the lipid; interaction with a phosphatidylcholine monolayer was considerably less. Locust apoLp-III binds with high affinity (Kd = 7.9.10(-9) M) to 1,2-diacylglycerol, which is consistent with its function in the LDLp particle; affinity for phosphatidylcholine is considerably lower. While the molecular area of locust apoLp-III in a monolayer is 2080 A2/molecule at the collapse pressure, in mixed monolayers of apoLp-III and lipid, the mean molecular area is decreased. Deglycosylation of the apoLp-III did not affect its interfacial stability. ApoLp-III from the moth Manduca sexta, which we included for comparison, demonstrated a similar reduction in molecular area resulting from interaction with lipid. These data do not support the hypothesis that interaction of apoLp-III with a lipid surface will lead to doubling of the molecular area of the protein (Kawooya, J.K., Meredith, S.C., Wells, M.A., Kézdy, F.J. and Law, J.H. (1986) J. Biol. Chem. 261, 13588-13591). The area of locust apoLp-III of 12.9 A2/amino acid residue at the collapse pressure is consistent with monolayers of alpha-helical proteins; circular dichroic spectra confirm a high alpha-helix content. The surface properties of apoLp-III reported here enable a high surface concentration of diacylglycerol in the LDLp particle, allowing the lipoprotein to act as an efficient reutilizable lipid shuttle.
Demel
R A
RA
Centre for Biomembranes, University of Utrecht, The Netherlands.
Van Doorn
J M
JM
Van der Horst
D J
DJ
D000818:Animals; D001053:Apolipoproteins / Q000737:chemistry / Q000302:isolation & purification*; D002942:Circular Dichroism; D004075:Diglycerides; D006110:Grasshoppers / Q000378:metabolism*; D010713:Phosphatidylcholines; D011487:Protein Conformation; D012996:Solutions; D013499:Surface Properties
D001053:Apolipoproteins; D004075:Diglycerides; D010713:Phosphatidylcholines; D012996:Solutions; C042470:apolipophorin III; C017251:1,2-oleoylphosphatidylcholine; C013965:diolein