Question

Match the following Actin Binding Protein (ABP) with its functional description:

1. Catalyzes the exchange of ADP for ATP on actin 2. Binds F-actin ADP severing the filament. 3. Bacterial surface protein that activates actin nucleating proteins. 4. Nucleating protein that results in branched actin filaments. 5. Nucleating protein that results in long, unbranched actin filaments 6. Binds and caps the positive (+) end of actin filaments inhibiting subunit addition 7. Acts as a buffer for monomeric actin by sequestering G-actin.

a. Arp2/3 complex b. Thymosin beta-4 c. CapZ d. Profilin e. ActA f. Cofilin g. Tropomodulin h. Formin i. Tropomysosin

Answer 1

1) Catalyze the exchange of ADP for ATP on actin	d) Profilin
2) Binds F-actin ADP severing the filament.	f) cofilin
3) Bacterial surface protein that activates actin nucleating proteins.	e) ActA
4) Nucleating proteins that result in branched actin filaments.	a) Arp2/3 complex
5) Nucleating proteins that results in long unbranched actin filaments.	h) Formin
6) Binds and caps the positive (+) end of actin filaments inhibiting subunit addition	c) CapZ
7) Acts as a buffer for monomeric actin by sequestering G-actin.	b) Thymosin beta -4

Explanation

1) Profilin  catalyzes the exchange of actin-bound ADP to ATP converting the poorly polymerizing ADP-actin monomers into readily polymerizing ATP-actin monomers.

2) Cofilin binds F-actin ADP severing the filment and accelerates the monomer dissociation from the pointed ends.

3)ActA is a bacterial surface protein which activates actin nucleating proteins like Arp2/3.

4) Arp2/3 complex- Arp2/3 is responsible for actin filament branching. Arp2/3 is a seven-protein complex having Arp2, Arp3, and five unique polypeptides. Arp2/3 is activated by WASP (Wisckott–Aldrich syndrome protein), which exists in an autoinhibited conformation. Binding of Cdc42 (GTP bound) and PI(4,5) P2relieves the inhibitory conformation of WASP and forms the Arp 2/3complex and also the actin filament branching takes place.

5) Formin actin binding protein helps in forming a long unbranched actin filament. Formins dimerize and form a hoop-shaped structure which acts as a ‘barbed end’ filament cap to stabilize the formation of an adjacent actin dimer. This new formin-bound actin filament is able to elongate by the insertion of ATP–actin monomers between the formin cap and the ‘barbed end’ of the filament.

6) CapZ is an actin binding protein which binds and caps the positive (+) end of actin filaments, inhibiting subunit addition.

7) Thymosinbeta-4 is a member of actin monomer-sequestering proteins,it acts as a buffer for monomeric action by sequestering G-actin.