Abstract 1377 Colocalization and dynamics of reflectin biomolecular condensates

Cephalopods possess reflectin proteins, which play a central role in the structure and function of iridescent nanostructures in specialized skin cells called iridocytes. Reflectin proteins are cationic block copolymers composed of highly conserved domains interspaced with lesser conversed cationic linkers. Tunability of iridescence in certain species of squid is hypothesized to be due to regulated, reversible formation of reflectin biomolecular condensates. Formation of biomolecular condensates in tunable iridocytes is associated with phosphorylation and dephosphorylation of different types of reflectin. To elucidate the mechanism of reflectin biomolecular condensates in eukaryotic cells, we examined colocalization when Doryteuthis opalescens (a squid species with tunable iridocytes) reflectin subtype A1 was coexpressed with subtypes A2, B, or C in HeLa or HEK293T cells. We also conducted FRAP experiments to evaluate the dynamics of different reflectin subtypes when expressed in HeLa cells. Lastly, we tested for the presence of reflectin phosphorylation of reflectins expressed in HeLa cells, either with or without the ionophore ionomycin. Confocal microscopy confirms that reflectins A1, A2, B, and C have distinct localization and self-assembly patterns. Coexpression experiments indicate that reflectin A1 associates with other subtypes to differing degrees, depending on coexpressed subtype, suggesting that reflectin subtypes have differing affinities for each other. FRAP measurements indicate that condensate dynamics vary significantly between different subtypes. This evidence supports the hypothesis that a complex, regulated interplay between different reflectin subtypes within tunable squid iridocytes may underlie these species' tunable, reversible iridescence. This work was supported by SUA and NSF grant 2233670 (to R Levenson).