The Structure and Function of Serum Opacity Factor: A Unique Streptococcal Virulence Determinant That Targets High-Density Lipoproteins

Figure 5

Model of the opacification reaction of SOF with high-density lipoproteins (HDLs). SOF initiates the opacification reaction by binding to HDL. SOF is a heterodivalent fusogen that crosslinks two or more HDL particles and simultaneously induces the release of free apo A-I and promotes the fusion of the resultant particles culminating in the formation of a cholesterol-ester rich microemulsion (CERM) and neo-HDL. Neo-HDL is deficient in free cholesterol/cholesterol esters and rich in phospholipids and apo A-II. Some HDL particles also contain apo E, which is preferentially retained in CERM. Structures are not drawn to scale. CERM particles range from 100 to 500 nm in size whereas HDL particles are ∼8.5 nm [38]. Current data indicates one CERM particle contains cholesterol-esters from ∼400,000 HDL particles. It is these CERM particles that cause serum to become opaque due to their large size and insolubility in aqueous solutions.