This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Streptococcal serum opacity factor (SOF), a virulence determinant that clouds serum, causes HDL instability. SOF appears to catalyzes transfer of the CE of ~100,000 HDL particles (D ~ 90 ¿) into a large CE-rich microemulsion (CERM; r >1,000 ¿) leaving a new HDL-like particle, neo HDL (D ~ 20¿), and releasing lipid-free(LF) apo A-I. rSOF is a heterodivalent fusogenic protein that uses a docking site to displace apo A-I and bind to exposed CE surfaces on HDL; the resulting rSOF-HDL complex recruits additional HDL with its binding delipidation site and, through multiple fusion steps, forms a CERM. By cryo-EM we hope to confirm this interpretation of SOF activity, and uncover more information about neo-HDL and CERMs.