Proteins

Question 1

DISC1

Answer 1

Disrupted-in-Schizophrenia-1

Found to be under expression in family with increased prevalence of schizophrenia and bipolar disorder

Scaffolding protein involved in;

• brain development
• neuronal plasticity
• affects TF activity, CAMP activation

E3 for DISC1 is called FBXW7 (F-box E3 ligase)
Inhibiting F-box decreases ubiquitination, increases levels of DISC1
- potential therapy for schizophrenia (requires inhibitor able to cross blood-brain barrier)

Question 2

P53/ Mdm2

Answer 2

It binds to de-phosphorylated P53; ubiquitinates it, and sends it to degradation.

As a response to cell damage/DNA damage, P53 is phosphorylated. This prevents degradation of it while it is needed.

SUMOylation of P53 prevents it from performing its transcriptional function
Ubiquitination of different residues leads to increase in P53 function or degradation

This is important in cancer protection (repair cell damage etc.)

Over-expression of MDM2 decreases P53 function (this increases risk of cancer)

Question 3

E3

Answer 3

There are four major classes of ubiquitin ligase: HECT domain proteins, U-box proteins, monomeric RING finger E3s, and multisubunit E3 complexes that contain a RING finger protein. a | HECT domain E3s use a unique mechanism in which ubiquitin (Ub) is transferred from an E2 to a conserved cysteine residue within the E3 via transthiolation and is then transferred from the E3 to a substrate amino group. All other types of E3 facilitate transfer of Ub from a charged E2 directly to a substrate. b | The RING finger motif comprises a Zn2+ binding domain that is required for E2 binding. The U-box also binds E2s and is structurally similar to the RING motif, but does not bind metal ions. Monomeric RING finger E3s and U-box E3s bind to both the substrate and the E2. In multimeric RING finger complexes, the RING finger protein binds the E2 while other proteins in the complex bind the substrate. These multimeric complexes include the anaphase promoting complex/cyclosome and cullin-RING ligases (CRLs). A unique aspect of CRLs is the requirement for modification of the cullin subunit by NEDD8 for ubiquitin ligase activity (Box 2). RBX, RING-box protein.

Question 4

Ubiquitin

Answer 4

a | Ubiquitin-activating enzyme (UAE) binds ATP and ubiquitin (Ub) to form a ternary complex consisting of E1–ubiquitin thioester with ubiquitin–AMP bound (see text for details). The thioester-bound ubiquitin is then passed to one of several E2 conjugating enzymes through a transthiolation reaction. The ubiquitin-charged E2 then forms a complex with an E3 ligase and a protein substrate to transfer ubiquitin to a lysine residue on the substrate. To mark substrates for degradation, multiple ubiquitins are similarly recruited to produce a K48-linked polyubiquitin chain. Following release from the E3, the proteasome recognizes the polyubiquitin chain, and the substrate is deubiquitylated and destroyed. Substrates marked with ubiquitin chains linked through lysines 6, 11, 27, 29 and 33 also seem to be primarily destined for degradation. Alternatively, substrates marked with monoubiquitin, K63 ubiquitin chains, or poly-ubiquitin chains using different ubiquitin lysine sites are involved in functions such as protein trafficking, signalling and enzyme activation.