Expression of human preproapo AI and pre(delta pro)apoAI in a murine pituitary cell line (AtT-20). A comparison of their intracellular compartmentalization and lipid affiliation.

The role of the NH2-terminal propeptide of human apolipoprotein (apo) AI in intracellular transport and lipid-protein interactions was examined by transfecting a murine anterior pituitary cell line (AtT-20) with the human preproapo AI gene and a mutant gene lacking the 18-base pair segment of exon III encoding its hexapeptide prosegment. ProapoAI was not processed to the mature apolipoprotein either prior to or after export from these cells making this an attractive model system for directly assessing structure/activity relationships of its propeptide. ApoAI was sorted into a regulated pathway for protein export. The signal responsible for this trafficking pattern was not contained in the prosegment since both pro- and mature ApoAI exhibited a similar rate of secretion, a similar magnitude of stimulation of export by the secretagogue 8-bromo-cyclic AMP, and similar targeting to dense core granules. This sorting behavior, exhibited by a protein which is not normally targeted to dense core secretory granules in its cells of origin, raises questions about the domains and mechanisms involved in protein sorting into the regulated and constitutive pathways of AtT-20 cells. Density gradient ultracentrifugation, immunoaffinity chromatographic and electron microscopic analysis indicated that approximately 10% of proapoAI and approximately 10% apoAI appeared in AtT-20 culture media in the form of two discrete classes of nascent lipoproteins: a small 6-8 nm spherical particle and a larger discoidal particle. These particles had morphologies identical with those secreted by Hep G2 cells which normally synthesize apolipoproteins. Although these results do not resolve the issue of whether or not a fraction of apoAI can act as an acceptor of cellular lipid during its transport through the secretory pathway, the data do show that this functional capability for lipoprotein assembly is not obviously regulated by its prosegment.