ii Draw a schematic diagram that shows a close-up view oftwo plasma membranes as
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ii Draw a schematic diagram that shows a close-up view oftwo plasma membranes as the come together during cell fusion. Show membrane proteins in both cells that were labeled from the outside by binding of differently colored fluorescent antibody molecules. Indicate in your drawing the fates of these color tags as the cells fuse. Will they remain on the outside of the hybrid cell after cell fusion and still be there after the mixing of membrane proteins that occurs during the incubation at 37oC? How would the experimental outcome be different if the incubation were done at 0°C? 12 Compare and contrast protein import into the ER and into the nucleus. List at least two major differences in the mechanisms, and speculate why the ER mechanism might not work for nuclear transport and vice versa. 13 some proteins shuttle back and forth between the nucleus and the cytosol. They need a nuclear export signal to get out ofthe nucleus. How do you suppose they get into the 14 Cell membranes are fluid, and thus proteins within the lipid bilayer. can diffuse la However, sometimes the needs to proteins to a particular membrane domain. cell three mechanisms that a cell can use to restrict protein to a particular place in the a membrane. 15 Band 3 protein is important for erythrocyte shape and cortical flexibility. It participates in multiprotein complexes that include ankyrin and spectrin. There are three major pulations of Band 3 in the plasma membrane: (1) unassociated with the cortex, with lateral diffusion limited only by spectrin fibers; (2) associated with spectrin fibers; and (3) associated with the actin junctional complex. Figure Q16Explanation / Answer
13. The double-layered nuclear membrane has 'holes’ called nuclear pore complexes (NPCs). Each NPC contains about 30-50 different proteins, known as nucleoporins, which form a physical passage through which molecules can be transported into and out of the nucleus. Most small molecules, such as ions and small proteins, can pass through the NPC by passive diffusion alone. However, larger molecules, such as proteins and RNA, must first associate with the nuclear transport receptors called karyopherins that specialise in transporting a particular molecule through the NPC. Karyopherins are of two types; importins which import molecules into nucleus and exportins which export molecules out of nucleus. Some karypherins can do both, import and export molecules.
The proteins which need to be imported into the nucleus contain a nuclear localisation signal (NLS) which is recognized and bound by the importins that then transport the cargo from the cytoplasm into the nucleus, through the NPC.
The importins are dimeric molecules consisting of importin beta which interacts with the nucleoporins of the NPC and sometimes with the molecule to be transported and importin alpha which acts as an adaptor molecule for importin beta and recognizes the NLS on the cargo.
14. The three ways that a cell can restrict the lateral diffusion of a protein and immobilize it on the cell membrane are
a) The membrane proteins can can form large aggregates that diffuse very slowly, e.g., bacteriorhodopsin in the purple membrane of Halobacterium.
b) The proteins can be tethered by interactions with macromolecular assemblies either outside or inside the cell.e.g., The proteins present in the membrane of RBCs are tethered to the cytoskeleton on the inside.
c) Or the membrane proteins can interact with proteins on the surface of adjacent cells.
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