Oligomerisation of pentraxin-3: Insights from cryoEM

Abstract

Pentraxin-3 (PTX3) is a secreted protein with roles in the stabilisation of hyaluronan-rich extracellular matrices involved in reproductive biology and inflammatory processes, as well as additional functions in innate immunity and cancer. Our recent structural studies (Shah et al., 2025; DOI:10.1016/j.matbio.2025.01.002), involving X-ray crystallography, cryo-electron microscopy (cryoEM) and AlphaFold modelling, have provided clues as to how PTX3 becomes assembled into an octamer from eight identical protomer subunits. Here it was proposed that four protomers initially form a tetramer, composed of a highly extended N-terminal region consisting of coiled-coil structures and C-terminal pentraxin domains, where two tetramers then immediately align and associate via an extensive network of salt bridges, allowing stabilisation of the octamer via the formation of disulphide bonds. However, a paper published around the same time provides an alternative perspective (Guo et al., 2025; DOI: 10.1016/j.ijbiomac.2024.139207). The authors propose, based on cryoEM analyses, that in addition to octamers, stable dimers, tetramers and hexamers of PTX3 can also assemble, where it is the dimers that provide the ‘building blocks’ for generation of the various oligomeric forms. In this commentary we suggest that the presence of dimers, tetramers and hexamers is likely an artefact of the construct used in recombinant expression, since the existence of these oligomers is not consistent with other studies on PTX3. We also provide a model to clarify how protomers become assembled into an octamer via sequential formation of a disulphide-linked tetramer, non-covalent association of two tetramers through aligned ionic interactions and the formation of disulphide bonds between the C-terminal pentraxin domains.