Mass photometry analysis revealed monomer-dimer equilibrium of the synaptic adhesion molecule neuroligin.

Neuroligin (Nlgn) is a post-synaptic adhesion molecule that regulates synaptic maturation through trans-synaptic interactions with pre-synaptic neurexins. The extracellular domain (ECD) of Nlgns is known to form dimers that are critical for their functions. However, the dynamic nature of this dimerization remains poorly understood due to technical limitations of conventional methods. In this study, we employed mass photometry to quantitatively evaluate the oligomerization states of recombinant human Nlgn1, Nlgn2, and Nlgn3 ECDs. Unlike size exclusion chromatography-multi angle light scattering, which confirmed a predominant dimeric state at the μM concentrations, MP detected transient Nlgn monomers in the nM range. Our results demonstrate that the Nlgn ECD exists in a concentration-dependent equilibrium between monomeric and dimeric forms. Notably, we identified differences in dimerization affinities among Nlgn family members. Among the isoforms tested, Nlgn3 exhibited the highest homodimerization affinity, followed by Nlgn2 and Nlgn1. These findings suggest that Nlgn dimerization is a dynamic process governed by isotype-specific affinities, likely arising from sequence variations at the dimer interface. Furthermore, this study highlights the utility of mass photometry in quantifying the equilibrium of protein oligomerization.