12.16:
Mechanisms of Membrane-bending
A membrane has a positive curvature when it bends towards the inner or cytoplasmic side. In contrast, a membrane has a negative curvature if it bends towards the outer or extracellular side.
There are multiple mechanisms that can cause membrane bending.
Lipid-induced membrane bending is due to the asymmetric aggregation of specific lipids in the membrane.
Lipids with large head groups induce positive membrane curvature, whereas lipids with small head groups produce a negative membrane curvature.
Protein scaffolding is another mechanism where proteins either deform the membrane or stabilize an already deformed membrane.
In a different mechanism, hydrophobic protein domains are reversibly inserted in one layer of the membrane to increase its area compared to the other layer. This asymmetry forces the membrane to bend.
Finally, the push and pull forces generated by the assembly and disassembly of the cytoskeleton and its associated motor proteins can also bend the membrane to form projections, such as filopodia in epithelial cells.
12.16:
Mechanisms of Membrane-bending
The living membranes are flexible due to their fluid mosaic nature; however, their bending into different shapes is an active process regulated by specific lipids and proteins. The membrane bending can be transient as seen in vesicles or stable for a long time as in microvilli. Cells regulate the size, location, and duration of the membrane curvature.
Membrane bending can happen due to intrinsic changes in lipid composition or extrinsic association with different proteins. The proteins involved in creating membrane asymmetry or clustering of specific lipids play an essential role in lipid-induced membrane bending. The shape of integral proteins can also affect the bending of the membrane. For example, conical or inverse-conical shaped transmembrane proteins produce bending in the membrane. The insertion of amphipathic proteins, such as epsin, in one of the layers also results in the bending of the membrane.
The curved proteins like Bin/amphiphysin/Rvs or BAR dimer proteins create positive membrane curvature, whereas the inverse BAR or I-BAR protein induces a negative curvature in the membrane. BAR proteins also insert their hydrophobic domain in the membrane, which further increases the curvature of the membrane. Besides the well-established mechanisms, protein crowding is a recently discovered mechanism for membrane bending where the lateral pressure generated by protein-protein interactions drives membrane bending. Cytoskeletons are also a significant player in membrane bending, and the forces generated by them are responsible for the morphology of some organelles like the endoplasmic reticulum and Golgi apparatus.
The cell organelles like the endoplasmic reticulum, Golgi apparatus, and mitochondria have intricately folded membranes; these foldings are essential for their functioning. Membrane bending is also necessary for membrane scission, membrane fusion, and membrane curvature sensing enzyme's activation. Mutations in genes that produce membrane-bending proteins such as myotubularin, amphiphysin-2, and dynamin-2 can lead to a group of neuromuscular disorders known as centronuclear myopathies.