  The biophysical analysis of dynamically formed multi-protein complexes in
solution presents a formidable technical challenge. Sedimentation velocity (SV)
analytical ultracentrifugation achieves strongly size-dependent hydrodynamic
resolution of different size species, and can be combined with multi-component
detection by exploiting different spectral properties or temporally modulated
signals from photoswitchable proteins. Coexisting complexes arising from self-
or hetero-associations that can be distinguished in SV allow measurement of
their stoichiometry, affinity, and cooperativity. However, assemblies that are
short-lived on the time-scale of sedimentation (t1/2 < 100 sec) will exhibit an
as of yet unexplored pattern of sedimentation boundaries governed by coupled
co-migration of the entire system. Here, we present a theory for
multi-component sedimentation of rapidly interacting systems, which reveals
simple underlying physical principles, offers a quantitative framework for
analysis, thereby extending the dynamic range of SV for studying
multi-component interactions
