Acidic low-complexity domain truncation modulates coacervation and cationic protein-induced morphological reorganization.

Protein coacervates are aqueous colloidal compartments, and their formation and interfacial organization depend on electrostatic and non-electrostatic interactions. In this study, we investigated how truncation of the acidic domain affects the apparent coacervation behavior and cationic protein-induced morphological reorganization of GG1234-derived synthetic anionic proteins. Three related constructs, GG1234, GG12-2, and GG12-1, were compared under controlled pH, ionic strength, and protein-mixing conditions. Turbidity measurements showed apparent simple-coacervation regimes for all three constructs under acidic conditions. Truncation shifted the apparent high-turbidity regime to higher pH and changed its non-monotonic dependence on ionic strength. In mixtures with lysozyme, a model cationic protein, the ratio-dependent turbidity profiles varied with both pH and construct length, which is consistent with an influence of truncation-associated changes in acidic residue content and chain length on the apparent ratio window of heterotypic coacervation. When bhBMP-2 was added to preformed simple coacervates, the interfacial reorganization was construct-dependent. GG1234 formed pronounced vesicle-like/core-shell structures (97.5%), GG12-2 showed partial reorganization (77.7%), and GG12-1 remained largely spherical (0%) under the tested formulation. Because truncation also removes Tyr- and Ile-containing motifs, these differences are interpreted as sequence-truncation effects rather than as effects of reduced acidic valency alone. These results show that acidic domain truncation affects both the pH- and salt-dependent simple- and complex-coacervation windows and the capacity for interfacial morphological reorganization. This coupling may also be applicable to other anionic intrinsically disordered proteins whose phase behavior is charge-dominated.