Mass-sensitive particle tracking (MSPT) to elucidate the membrane-associated MinDE reaction cycle

In spite of their great importance in biology, methods providing access to spontaneous molecular interactions with and on biological membranes have been sparse. So far, it has been consensus that their observation with sufficient sensitivity and time resolution requires the introduction of - predominantly fluorescent-labels to the system. However, the recent advent of mass photometry to quantify mass distributions of unlabelled biomolecules landing on surfaces raised hopes that this approach could be transferred to membranes. Here, we introduce mass-sensitive particle tracking (MSPT), enabling simultaneous label-free tracking and monitoring of molecular masses of single biomolecules diffusing on lipid membranes. We applied this approach to the highly non-linear reaction cycles underlying MinDE protein self-organisation. MSPT allowed us to determine the stoichiometry and turnover of individual membrane-bound MinD/MinDE protein complexes and to quantify their size-dependent diffusion. We found that MinD assembles into complexes larger than the commonly postulated dimer, through lateral interactions of membrane-bound complexes and subunit recruitment from solution. Furthermore, the ATPase-activating protein MinE interconnects MinD into high-molecular-weight heteromeric complexes and affects their subunit turnover and concerted membrane release. This study demonstrates the potential of MSPT to enhance our quantitative understanding of both prokaryotic and eukaryotic membrane-associated biological systems. ### Competing Interest Statement The authors have declared no competing interest. * (ATP) : adenosine-5’-triphosphate (ADP) : adenosine-5’-diphosphate (BSA) : bovine serum albumin (18:1 Biotinyl Cap PE) : 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-cap biotinyl (DOPC) : dioleoyl-sn-glycero-3-phosphocholine (DOPG) : dioleoyl-sn-glycero-3-phosphoglycerol (HRP) : horseradish peroxidase (iSCAT) : interferometric scattering (P-loop) : phosphate binding loop (PSF) : point spread function (SPT) : single-particle tracking (SUV) : small unilamellar vesicle (SLB) : supported lipid bilayer (MP) : mass photometry (MSPT) : mass-sensitive particle tracking (MTS) : membrane targeting sequence