Serum Amyloid A 1.1 Forms a Conformational Temperature Sensor at the Lipid Surface

Abstract

Serum amyloid A (SAA) is a natively unfolded acute-phase protein. Normally SAA circulates on plasma high-density lipoproteins (HDL) whose major protein is apoA-I. In inflammation SAA levels can increase 1000-fold and it becomes the major plasma protein that displaces up to 50% of apoA-I from HDL, forming SAA-HDL with altered metabolic properties. To determine whether SAA alters structural stability of HDL, human HDL were incubated with murine SAA 1.1 to obtain SAA-HDL. Calorimetric, spectroscopic and biochemical studies showed that HDL fusion and rupture, which were characterized in our previous work, occur at similar temperatures in normal HDL and in SAA-HDL. Therefore, the exchangeable fraction of apoA-I has little effect on HDL stability which, we propose, is determined by the apoA-I fraction that cannot be displaced. This apoA-I likely forms an antiparallel double-belt dimer around HDL. Our results argue against domain swapping among apoA-I molecules on HDL, which is important for HDL structure and function. Surprisingly, our results reveal helical unfolding in SAA-HDL circa 38°C. This novel transition was observed only in samples containing excess SAA and the lipid surface, but neither in lipid-free SAA nor in isolated SAA-HDL containing lipoproteins alone. The transition was invariant in a wide range of solvent conditions (pH 5.5-8.5, 10-150 mMol Na salt), upon lipolysis by PLA2, or upon lipoprotein rupture. In sum, SAA near the lipid surface undergoes folding/unfolding at physiologic temperatures. We propose that excess SAA forms a local conformational temperature sensor and suggest a new function for SAA in acute phase response. Funded by NIH GM067260.