Intracellular and extracellular processing of human apolipoprotein A-I: secreted apolipoprotein A-I isoprotein 2 is a propeptide.

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Abstract
It was recently proposed that the major secreted isoprotein from of human aplipoprotein A-I (designated apo A-I2) is modified extracellularly to become the predominant apo A-I form seen in plasma (designated apo A-I4). The primary translation product of human apo A-I (designated apo A-I2p) has a 24-amino-acid NH2-terminal extension with a sequence of Met-Lys-Ala-Ala-Val-Leu-Thr-Leu-Ala-Val-Leu-Phe-Leu-Thr-Gly-Ser-Gln-Ala-Arg-His-Phe-Trp-Gln-Gln. The first 18 amino acids of this NH2-terminal extension are cleaved intracellularly by the signal peptidase, resulting in the formation of apo A-I2, which is the secreted form of apo A-I. Sequence analysis of apo A-I2 confirmed that it contains a hexapeptide extension at its NH2 terminus compared to apo A-I4, demonstrating that apo A-I2 is a propeptide and that the apo A-I2 to apo A-I4 conversion involves the removal of the NH2-terminal hexapeptide of apo A-I2 by a protease in plasma, lymph or both. Apo A-I is probably synthesized as a prepropeptide, which undergoes intracellular and extracellular proteolysis to attain the major plasma apo A-I4 isoprotein form.

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