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A biophysical measurement relates an observed signal to a physical property of a biological system, such as diffusion, force, molecular abundance or membrane potential. The inference requires a measurement model and calibration: a brighter fluorescent spot can reflect more labelled molecules, a change in brightness per label or background. Independent controls help separate these possibilities. Single-molecule measurements can reveal heterogeneous states that an average conceals, while the recorded trajectory remains limited by spatial and temporal resolution. ^[ref]

A biophysicist who coauthored single-molecule methods for estimating the number of subunits in membrane-bound proteins. Counting bleaching steps connected an optical time series to molecular assembly.^[ref]

Biophysicist whose work includes counting subunits of membrane-bound proteins through fluorescence measurements. With collaborators, Ulbrich connected observed signal changes with the number of labeled molecular components, making labeling efficiency part of the interpretation.^[ref]

Protein oligomerization associates several protein subunits into a defined assembly, such as a protein dimer. It can create cooperative binding sites, regulate activity or build structural units. A measured assembly depends on concentration and conditions: a dimer can dissociate on dilution. Determining subunit count therefore requires an assay model as well as a measured size or signal. ^[ref]

A protein subunit is an individual polypeptide component of a protein complex. The number of protein subunits counts chain copies, whereas the number of subunit types counts distinct kinds of chain. An assembly $A_2{B}_2$ has four protein subunits of two types. A protein domain is a region within a chain and need not be a separate protein subunit; the arrangement of several chains belongs to protein quaternary structure.

The alpha/beta tubulin heterodimer illustrates the distinction: it contains two related but different polypeptides, not two domains cut out of one chain. The 1998 electron-crystallographic model resolved the alpha/beta architecture at $3.7$ angstrom resolution, using zinc-induced tubulin sheets.^[ref]

Subunit counting can also become an optical experiment. In total internal reflection fluorescence microscopy, tagged membrane proteins can be observed as isolated fluorescent spots; successive photobleaching steps reveal individual fluorescent tags disappearing. Subunit counting in membrane-bound proteins by Maximilian H. Ulbrich and Ehud Y. Isacoff used this approach on membrane proteins expressed in frog oocytes. For NR1/NR3B receptor assemblies, tagging one component type mainly gave two steps, while tagging both gave four, supporting a two-plus-two arrangement.^[ref]

A count of visible steps need not equal the true number of protein subunits: some tags fail to fluoresce, and two tags may bleach too close together to resolve. The experiment therefore combines step counts with a model of labeling and detection, rather than interpreting every two-step trace as proof of a two-chain protein complex.^[ref]