Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I), 217 (apoA-I), and 226 (apoA-I) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 ± 1.28/day (±S.D.) for apoA-I, 6.34 ± 0.81/day for apoA-I, and 4.42 ± 0.51/day for apoA-I were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 ± 0.07/day) and h-apoA-I (0.91 ± 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL and HDL. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre- position, but not within the position as did r-apoA-I. These results indicate that the carboxylterminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

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