Deletion of the propeptide of apolipoprotein A-I impairs exit of nascent apolipoprotein A-I from the endoplasmic reticulum.
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.
McLeod
R S
RS
Department of Pathology, University of British Columbia, Vancouver, Canada.
Robbins
C
C
Burns
A
A
Yao
Z
Z
Pritchard
P H
PH
D000595:Amino Acid Sequence; D000818:Animals; D016632:Apolipoprotein A-I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001054:Apolipoproteins A / Q000737:chemistry / Q000378:metabolism*; D001483:Base Sequence; D002460:Cell Line; D006224:Cricetinae; D018076:DNA, Complementary / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D018076:DNA, Complementary; D011498:Protein Precursors; D011994:Recombinant Proteins