Resolving oligomeric states of photoactivatable proteins in living cells via photon counting histogram analysis

Oligomerization of photoactivatable proteins underlies many optogenetic strategies, yet their assembly states remain difficult to quantify in living cells. Here, we applied photon counting histogram analysis to directly measure the oligomerization of widely used optogenetic modules, <i>Vaucheria frigida</i> Aureochrome light-oxygen-voltage (VfAuLOV) and <i>Arabidopsis</i> <i>thaliana</i> cryptochrome 2 (AtCRY2), in living HEK293T cells. Oligomerization of both photoactivatable protein variants is concentration-dependent in cells. VfAuLOV primarily forms dimers, whereas AtCRY2 transitions into tetramers at concentrations above 1,000 nM, consistent with cryoEM structures. Human CRY2 exhibits light-independent oligomerization, while inactive AtCRY2 mutants (D387A and R439L) remain monomeric in light or darkness. Surprisingly, the constitutively active AtCRY2(W374) mutant still undergoes light-mediated oligomerization. The extent of light-induced lytic cell death correlates with the oligomerization state of these proteins when fused to receptor-interacting serine/threonine protein kinase 3. This study establishes a quantitative framework to resolve protein assembly dynamics in living cells, advancing mechanistic understanding of optogenetic tools and broadening their applications in cell signaling research.