Intracellular early and late modifications of human apolipoprotein A-II. Effect of glutamine- +1 to leucine substitution.
It has been shown previously that apoA-II undergoes several intracellular modifications in HepG2 cells (Hussain & Zannis, 1990). In the present study, we have generated permanent cell lines in mouse C127 cells which express the normal apoA-II gene and a mutated form in which Gln+1 was substituted with Leu (Leu+1). This modification was designed to prevent cyclization of the N-terminal glutamine of apoA-II and thus identify the isoproteins which are precursors and products of the N-terminal cyclization reaction. The C127-expression cells were also utilized to study the cellular compartments where the apoA-II modifications occur as well as the importance of the modifications for apoA-II trafficking and secretion. We have found that apoA-II (Gln+1) synthesized by C127 and HepG2 cells had similar isoproteins. In both cell types, unmodified pro-apoA-II, designated isoprotein 3, had a similar isoelectric point as the cell-free translation product of apoA-II mRNA, suggesting that isoprotein 3 results from cleavage of the signal peptide. Isoprotein 3 represents an unmodified apoA-II isoprotein and undergoes an early modification into a more acidic isoprotein 1, which differs from isoprotein 3 by two negative charges. Brefeldin A treatment of the cells did not prevent the formation of isoprotein 1, suggesting that this modification occurs in a pre-Golgi compartment. Neuraminidase treatment of secreted apoA-II isoproteins did not affect isoprotein 1, indicating that it is not sialylated isoprotein. Isoprotein 1 undergoes further modifications which are consistent with cleavage of the propeptide, N-terminal cyclization and sialylation most likely resulting from O-glycosylation.(ABSTRACT TRUNCATED AT 250 WORDS)
Lopez
J
J
URA CNRS 1283, Institut des Cordeliers, Paris, France.
Roghani
A
A
Bertrand
J
J
Zanni
E
E
Kalopissis
A
A
Zannis
V I
VI
Chambaz
J
J
D016633:Apolipoprotein A-II / Q000378:metabolism*; D015180:Electrophoresis, Gel, Two-Dimensional; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D016297:Mutagenesis, Site-Directed; D009439:Neuraminidase / Q000494:pharmacology; D011499:Protein Processing, Post-Translational; D011994:Recombinant Proteins; D013329:Structure-Activity Relationship
D016633:Apolipoprotein A-II; D011994:Recombinant Proteins; D009439:Neuraminidase