STRUCTURAL RELATIONSHIPS BETWEEN NASCENT APOA-I-CONTAINING PARTICLES THAT ARE EXTRACELLULARLY ASSEMBLED IN CELL-CULTURE

Apolipoprotein A-I (apoA-I) incubated with CHO cells assembles three m ajor nascent lipid complexes two diameters of 7.3, 9, and 11 nm. Previ ous studies suggested that the smaller nascent particles were precurso rs for the larger nascent ones. To test this hypothesis, the 7.3, 9, a nd 11 nm apoA-I-lipid complexes formed by incubating CHO cells with li pid-free apoA-I were isolated and subsequently each subpopulation was re-incubated with cells in the absence of other subpopulations. The ph ysical-chemical characteristics of each subpopulation were examined be fore and after re-incubation in an effort to understand relationships; if any, between the different nascent complexes. The 7.3, 9, and 11 n m complexes were unique in that each of the particles had pre-alpha mo bility on agarose gels; this rapid migration was not altered by re-inc ubation with cells. Protein crosslinking studies indicated that the 7. 3, 9, and 11 nm complexes possessed 2, 3, and 4 apoA-I molecules per c omplex, respectively; it is unlikely that the size of the particle and number of apoA-I molecules per particle played a role in the increase d negative charge of the particles. The present study shows that small er particles did not give rise to larger ones upon re-incubation with cells. Rather, the 11 and 9 nm particles both generated smaller discs (the 11 nm giving rise primarily to 9 nm discs and the 9 nm complex gi ving rise to 7.3 nm discs) suggesting that, during incubation with cel ls, the complexes are destabilized and remodeled into smaller, not lar ger, complexes. Surprisingly, the 7.3 nm complexes during re-incubatio n with cells were extremely stable and did not undergo size alteration . When the 7.3 nm particles were incubated with additional small quant ities of lipid-free apoA-I (1-2 mu g/ml), larger discoidal complexes w ere generated suggesting that the formation of larger particles may be driven by the availability of lipid-free apoA-I;-Forte, T. M., J. K., Bielicki, L. Knoff, and M. R. McCall. Structural relationships betwee n nascent apoA-1-containing particles that are extracellularly assembl ed in cell culture.