Review - Review Questions from students: 1: Why are heat...

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Review 1 F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved. Questions from students: 1: Why are heat shock proteins considered "bad" enzymes? 2: In mitochondrial protein import, why when Hsp70 binds to ADP = high affinity for substrate and vice versa? 3: Please explain one more time the difference between necessary and sufficient of a sorting signal (3 requests) 4: Nuclear import and export: mechanism of importin vs exportin, recycling of Ran-GTPase 5: Process of protein quality control 6: How do Rabs direct transport? From my understanding, they aid in Rab effector binding, e.g. snares. Does this mean that Rabs are on all membranes that need fusion? Without Rabs, would membrane fusion be blocked?
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2 F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved. 1: Why are heat shock proteins considered "bad" enzymes? An enzyme is a protein that catalyzes an energetically favorable reaction, and speeds it up. I talked about a number of sets of proteins as “bad” enzymes, in the sense that without help, they do not go through their enzymatic cycles very well. That is, the “get stuck” at certain places in the cycle, and need a co-factor to help them continue. HSP70: a) when binds to ATP, it tends to wait in an ATP bound state, rather than rapidly converting ATP ADP. b) when binds substrate briefly, this promotes ATP hydrolysis, so substrate binding ACTIVATES enzymatic activity of HSP70 c) When HSP70 is bound to ADP, it is not in any hurry to release ADP d) BAG-1 is an exchange factor, promotes the release of ADP. Rabs: Inactive, in GDP-bound state. GEFs promote release of GDP, and then GTP can subsequently rebind. GAPs promote hydrolysis of GTP.
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3 F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved. 2: In mitochondrial protein import, why when Hsp70 binds to ADP = high affinity for substrate and vice versa? HSP proteins have a “substrate binding domain” (SBD) that binds to the target protein, and a “nucleotide binding domain” (NBD) that binds to the nucleotide (ATP or ADP) and acts as an ATPase. Binding of ATP induces closing of the NBD around the nucleotide, which induces a corresponding opening of the SBD allowing protein substrate release. If interested, can look at Flaherty et al, “3-dimensional structure of the ATPase fragment of a 70 K heat shock cognate protein” Nature 346, 623-628, 1990.
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F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved. 3: Please explain one more time the difference between necessary and sufficient of a sorting signal (3 requests) Necessary means that it is required. Something can be required, but insufficient. Example: for a functional signal sequence to get to inner mitochondrial membrane, you need 2 sequences –the first to get you to the mitochondria, the second to get you into the inner membrane. Neither is sufficient by themselves, but both are necessary. A signal is
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This note was uploaded on 12/13/2011 for the course BIOSCI d103 taught by Professor Staff during the Fall '11 term at UC Irvine.

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Review - Review Questions from students: 1: Why are heat...

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