Deletion of the propeptide of apolipoprotein A-I impairs exit of nascent apolipoprotein A-I from the endoplasmic reticulum

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Abstract
Human apolipoprotein (apo) A-I is secreted as a proprotein of 249 amino acids and is processed extracellularly to the mature form (243 amino acids) by removal of a six-residue propeptide segment. We have examined the role of the apoA-I propeptide in intracellular transport and secretion using transfected baby hamster kidney cells that secreted either proapoA-I (from the wild-type cDNA, A-Iwt) or mature-form apoA-I (from A-I delta pro, a cDNA in which the propeptide sequence was deleted). Deletion of the propeptide from the apoA-I sequence did not affect the rate of apoA-I synthesis, nor did it affect the fidelity of proteolytic removal of the prepeptide. However, the propeptide deletion caused mature-form apoA-I to accumulate within the cells as determined by pulse-chase experiments; the intracellular retention times for the mature-form apoA-I in which the propeptide was prematurely removed was three times longer than that of proapoA-I (t1/2 > 3 h compared with approximately 50 min). There was no detectable degradation of either form of newly synthesized apoA-I. Immunofluorescence microscopy revealed that, whereas the proapoA-I was located predominantly in the Golgi apparatus, large quantities of the mature-form apoA-I were detected in the endoplasmic reticulum and very little was in the Golgi apparatus of A-I delta pro-transfected cells. These findings suggest that the propeptide sequence may be involved in the intracellular transport of apoA-I from the endoplasmic reticulum to the Golgi apparatus. We propose that the function of the propeptide sequence is to facilitate efficient transport of apoA-I through the secretory pathway.