Deletion of central alpha-helices in human apolipoprotein A-I: Effect on phospholipid association

In order to better understand the structure-function properties of apolipoprotein (apo) A-I, we have constructed and expressed three apoA-I mutants using a system previously described for the expression of human apolipoprotein A-I (Rec.-apoA-I). These mutants (corresponding to deletion of apoA-I residues 100-143, 122-165, 144-286) have been studied for their ability to form reconstituted apoA-I-containing lipoproteins (LpA-I) with POPC and DMPC, and for their structural and physical properties. Rec.- and native apoA-I can form homogeneous discoidal Lp2A-I over a wide range of POPC/apoA-I ratios [(20-130)/1] and exhibit sizes ranging from 9.5 to 10.5 nm. When recombined with varying POPC content [(20-130)/1, POPC/A-I)], the three mutants produce homogeneous discoidal Lp2A-I that contain a low POPC/A-I molar ratio [(20-40)/1 for all mutants] and exhibit a nearly constant size [7.5-7.6 nm for Delta(100-143) and 7.9-8.0 nm for the other two mutants]. Kinetics of association of these proteins with DMPC are similar for A(100-143) and Rec.-apoA-I (t(1/2) of 4.0 and 4.4 min, respectively) but appear significantly reduced for Delta(122-265) and Delta(144-186) (t(1/2) of 7.5 and 6.9 min, respectively). While in the lipid-free form, all proteins have a similar thermodynamic stability with a very comparable free energy of unfolding (Delta G(D) degrees) for the alpha-helical structure, as determined by isothermal denaturation studies, Delta(100-143) has a significantly lower alpha-helical content (33%) as compared to the other proteins [40, 41, and 45% for Rec.-apoA-I, Delta(122-165), and Delta(144-186), respectively]. When associated to POPC, Delta(122-165) and Delta(144-186) have a higher alpha-helicity (63 and 63%) and an enhanced stability (2.5 and 2.3 kcal/mol, respectively) as compared to Delta(100-143) (49% and 1.8 kcal/mol) and Rec.-apoA-I (52% and 1.9 kcal/mol). These results suggest that the amphipathic alpha-helices within residues 100-186 are directly involved in interactions with phospholipids. The helical region 100-121 appears to be more important to the stabilization of the lipid-apoprotein complex formed whereas helices within residues 122-186 appear to be critical to the initial rates of association of the apoprotein with DMPC. These data suggest that an important role of the central domain 100-186 may be to maintain the plasticity of apoA-I and its ability to form different classes of HDL particles. Therefore, it is likely that this region may also play an important role in the functional properties of this apoprotein.