May have a role in chylomicrons and VLDL secretion and catabolism. Required for efficient activation of lipoprotein lipase by ApoC-II; potent activator of LCAT. Apoa-IV is a major component of HDL and chylomicrons (By similarity). Homodimer. Secreted in plasma. Nine of the thirteen 22-amino acid tandem repeats (each 22-mer is actually a tandem array of two, A and B, related 11-mers) occurring in this sequence are predicted to be highly alpha-helical, and many of these helices are amphipathic. They may therefore serve as lipid-binding domains with lecithin:cholesterol acyltransferase (LCAT) activating abilities (By similarity). Belongs to the apolipoprotein A1/A4/E family.