Membrane bending by protein-protein crowding.
Curved membranes are an essential feature of dynamic cellular structures, including endocytic pits, filopodia protrusions and most organelles. It has been proposed that specialized proteins induce curvature by binding to membranes through two primary mechanisms: membrane scaffolding by curved proteins or complexes; and insertion of wedge-like amphipathic helices into the membrane. Recent computational studies have raised questions about the efficiency of the helix-insertion mechanism, predicting that proteins must cover nearly 100% of the membrane surface to generate high curvature, an improbable physiological situation. Thus, at present, we lack a sufficient physical explanation of how protein attachment bends membranes efficiently. On the basis of studies of epsin1 and AP180, proteins involved in clathrin-mediated endocytosis, we propose a third general mechanism for bending fluid cellular membranes: protein-protein crowding. By correlating membrane tubulation with measurements of protein densities on membrane surfaces, we demonstrate that lateral pressure generated by collisions between bound proteins drives bending. Whether proteins attach by inserting a helix or by binding lipid heads with an engineered tag, protein coverage above ~20% is sufficient to bend membranes. Consistent with this crowding mechanism, we find that even proteins unrelated to membrane curvature, such as green fluorescent protein (GFP), can bend membranes when sufficiently concentrated. These findings demonstrate a highly efficient mechanism by which the crowded protein environment on the surface of cellular membranes can contribute to membrane shape change.
The University of Texas at Austin, Department of Biomedical Engineering, Austin, Texas 78712, USA. jcstach@austin.utexas.edu
Jeanne C
JC
Stachowiak
Eva M
EM
Schmid
Christopher J
CJ
Ryan
Hyoung Sook
HS
Ann
Darryl Y
DY
Sasaki
Michael B
MB
Sherman
Phillip L
PL
Geissler
Daniel A
DA
Fletcher
Carl C
CC
Hayden
D033942:Adaptor Proteins, Vesicular Transport / Q000378:metabolism; D000595:Amino Acid Sequence; D002462:Cell Membrane / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D034041:Monomeric Clathrin Assembly Proteins / Q000378:metabolism; D011485:Protein Binding; D054730:Protein Interaction Domains and Motifs; D017433:Protein Structure, Secondary
D033942:Adaptor Proteins, Vesicular Transport; D008565:Membrane Proteins; D034041:Monomeric Clathrin Assembly Proteins; C439023:clathrin assembly protein AP180; C114154:epsin