CaMKII nucleates an osmotic protein supercomplex to induce cellular bleb expansion

Blebs are membrane protrusions formed when localized regions of the plasma membrane detach from the actin cortex, enabling outward expansion driven by intracellular pressure. These structures play critical roles in cell migration and proliferation. While cortical actin contraction has been proposed as the primary driver of cytoplasmic fluid influx during bleb expansion, our prior observations revealed compartmentalization of Ca²⁺ ions and specific proteins (e.g., Mena) within expanding blebs. The functional significance of these components remained unresolved. In this study, we demonstrate that elevated Ca²⁺ levels during bleb expansion induce the assembly of a protein superstructure built around the CaMKII holoenzyme, incorporating Mena and other regulatory proteins. This complex exhibits intrinsic osmotic activity, facilitating water influx and directly contributing to bleb expansion. These findings elucidate a novel mechanism underlying bleb expansion and provide new insights into the dynamic regulation of physicochemical properties of the cytoplasm.