CAP1/CAP2 at the Actin Filament Barbed End: Reconciling Kinetic, Single-Molecule, Structural and Cellular Evidence

Cyclase-associated protein (CAP) is among the most abundant actin regulators in animal cells and one of the few acting at both filament ends. Its partnership with cofilin at the pointed end is now understood in structural detail. Its role at the barbed end is not. Between 2023 and 2026, four laboratories examined CAP at filament ends using microfluidics-assisted single-filament kinetics, three-color single-molecule imaging, cryo-electron microscopy and high-speed atomic force microscopy, and reached accounts read as contradictory. On the contested points they largely do not cite one another. Here we place these results in a common frame and separate what each method can establish from what it cannot. The two barbed-end studies agree more than is acknowledged, and their remaining disagreements fall into three categories. One is explained by filament-aging protocol, one open but with a clear methodological hierarchy, and one, the requirement for the CARP domain, not resolvable from published constructs. We then convert the mechanism into four constraints that a cellular claim must satisfy to be interpretable, and a fifth we propose rather than establish. Applying them to the disease literature suggests a provisional two-versus-two pattern across tissues rather than random heterogeneity, identifying one axis to be explained.