Figure 1. Proteins and lipid microdomains in membrane budding.

From left to right, A. Budding of phase separated lipid microdomains from GUVs composed of synthetic lipids. Reproduced by permission from (Baumgart et al., 2003), an example of membrane budding in the absence of any proteins. B. Shiga toxin (gold label) induces membrane buds acting from outside the plasma membrane. Reproduced by permission from (Romer et al., 2007) and example of a protein triggering a primarily microdomain-driven budding event. C. Caveolae, a hybrid between a membrane microdomain and a protein coat. Reproduced by permission from (Parton and Simons, 2007). D. ESCRT-I and – II induced buds in synthetic GUVs. Reproduced by permission from (Wollert and Hurley, 2010). Proteins organize these structures, but do not form a coat, suggesting a possible role for microdomains. E. HIV-1 buds visualized by EM tomography (Carlson et al., 2010). The bud is organized by the HIV-1 capsid protein, heavily enriched in raft lipids, and cleaved by ESCRT proteins. F. Deep etch visualization of clathrin coated pits. Reproduced by permission from (Heuser et al., 1987). Clathrin assembles into baskets in the absence of membranes, but is too flexible to deform membranes on its own. For this, clathrin needs help from other membrane-deforming proteins and possibly from lipids. G. Molecular model of the COP II cage. Reproduced by permission from (Russell and Stagg, 2010), an example of a protein structure that can form in the absence of lipids and can impose its shape on any simple bilayer-forming lipid mixture.