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Sar1 protein
COPII-recruitment GTPase that facilitates the unidirectionaltransfer of COPII vesicles from the ER membrane to the Golgi membrane.
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Coat-recruitment GTPases
Coat-recruitment GTPases are members of a family of monomeric GTPases.They include the ARF proteins,and the Sar1 protein,
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ARF proteins
ARF proteins, which are responsible for the assembly of bothCOPI and clathrin coats assembly at Golgi membranes
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The COPII subunits—probably in association with another protein—act as the GAP, stimulating hydrolysis of GTP by Sar1. Thus, the GAP is associated
with the vesicle membrane, not the Golgi membrane. Sar1–GDP then causes disassembly of the coat almost as soon as the vesicle has formed. The uncoated vesicle carries other proteins— a Rab protein and a vSNARE—that orchestrate docking and fusion with the Golgi membrane.
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Rab protein
- and Rab effectors direct the vesicle to specific spots on the correct target membrane.
- -play a central part in the specificity of vesicle transport.
- -monomeric GTPases.
- -Rab protein is associated withone or more membrane-enclosed organelles of the secretory or endocytic pathways,and each of these organelles has at least one Rab protein on its cytosolic surface
- -Rab proteins can function on transport vesicles, on target membranes, or both.
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Rab effectors
Once in the GTP-bound state and membrane-bound through anow-exposed lipid anchor, Rab proteins bind to other proteins, called Rab effectors,which are the downstream mediators of vesicle transport, membrane tethering,and membrane fusion
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SNARE proteins
- catalyze the membranefusion reactions in vesicle transport.
- These transmembrane proteins exist as complementary sets, withv-SNAREs usually found on vesicle membranes and t-SNAREs usually found ontarget membranes
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KDEL receptor
soluble ER resident proteins must bind to specializedreceptor proteins such as the KDEL receptor
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The receptor must have a high affinity for the KDEL sequence in vesicular tubularclusters and the Golgi apparatus, so as to capture escaped, soluble ER residentproteins that are present there at low concentration.
- It must have a low affinity forthe KDEL sequence in the ER, however, to unload its cargo in spite of the very highconcentration of KDEL-containing soluble resident proteins in the ER.
- -Golgi apparatus is slightly more acidic than that of the ER, which is neutral.
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The assembly anddisassembly of a clathrin coat
> Clathrin was omitted
B) Procaryotic membrane fragments were used Think and self-test yourself on omitting the other ingredients.
- A) Without clathrin, adaptor proteins still bind to receptors in the membrane, but no clathrin coat can form. Thus, no clathrin-coated pits or vesicles will form.
- B) Procaryotic cells do not perform endocytosis. A procaryotic cell does not contain any receptors with appropriate cytosolic tails that could mediate the binding of adaptor proteins. Therefore, no clathrin-coated vesicles will form
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effector proteins
Cells with identical receptors can respond differently to the same signal molecule because of differences in the internal machinery to which the receptors are coupled. Even when the entire signaling pathway is the same, cells can respond differently if they express different effector proteins at the ends of the pathways.
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