Many lipid-tethered proteins and glycolipids exist as monomers and nanoclusters on the surface of living cells. The spatial distribution and dynamics of formation and breakup of nanoclusters does not reflect thermal and chemical equilibrium and is controlled by active remodeling of the underlying cortical actin. We propose a model for nanoclustering based on active hydrodynamics, wherein cell surface molecules bound to dynamic actin are actively driven to form transient clusters. This consistently explains all of our experimental observations. Using FCS and TIRF microscopy, we provide evidence for the existence of short, dynamic, polymerizing actin filaments at the cortex, a key assumption of the theoretical framework. Our theory predicts that lipid-anchored proteins that interact with dynamic actin must exhibit anomalous concentration fluctuations, and a cell membrane protein capable of binding directly to actin can form nanoclusters. These we confirm experimentally, providing an active mechanism for molecular organization and its spatiotemporal regulation on the plasma membrane.

of the protein called glycosylphosphatidylinositol (GPI)-anchored proteins. The basic structure of a GPI-anchored protein consists of phosphatidylinositol (PI) linked to an unusual non-N -acetyl glucosamine, which, in turn, is linked to three mannose residues followed by an ethanolamine covalently linked to the protein via an amide linkage (EtNP-6Man ␣ 1-2Man ␣ 1-6Man ␣ 1-4GlcN ␣ 1-6 myo inositolphospholipid, Fig. 1A ). Depending on the species and functional context, there may exist variations in the side chain associated with the glycan core. These have been summarized in ( 1 ). The lipid moiety is necessary for the incorporation of GPI-anchored proteins into so-called lipid rafts/microdomains ( 2, 3 ), which can serve as a sorting station for a number of cell signaling molecules, thereby functioning as a reaction center. GPI-anchored proteins can exist in different forms depending on the context and the tissue in which they are expressed. Alternate splicing can cause the same protein to exhibit TM, soluble, or GPI-anchored forms; for example, neural cell adhesion molecule (NCAM) can exist in its GPI-anchored and soluble form when expressed in muscles; whereas, it takes up a TM form instead of the soluble form in brain. GPI anchoring of proteins occurs at the luminal face of The plasma membrane of the cell is comprised of a diverse set of proteins, many of which are transmembrane (TM) proteins spanning the entire bilayer, but a significant proportion of proteins are also lipid tethered, containing a complex glycan core attached to the C terminus

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Membrane proteins that couple to cortical actin show temperature-independent diffusion. The loss of this coupling and perturbation of cortical actomyosin dynamics render the diffusion temperature dependent. The findings suggest that active fluctuations arising from dynamic actin filaments at the cortex can drive diffusion on the cell membrane.