Two large classes of GTPase switch proteins are used in signal transduction pathways:

1. Heterotrimeric G proteins

2. Monomeric G proteins

TLS Online TPP Program

#Id: 8315

#Unit 2. Cellular Organization

Actin can reversibly assemble into filaments that consist of two helices of actin subunits

TLS Online TPP Program

#Id: 8316

#Unit 2. Cellular Organization

Persistence length,

minimum filament length at which random thermal fluctuations are likely to cause it to bend.

TLS Online TPP Program

#Id: 8317

#Unit 2. Cellular Organization

Actin polymerization in vitro proceeds in three steps:

1.Nucleation phase

2.Elongation phase

3.Steady-state

TLS Online TPP Program

#Id: 8318

#Unit 2. Cellular Organization

Nucleation phase 

Two G-actin molecules bind relatively weakly to each other, but addition of a third actin monomer to form a trimer makes the entire group more stable, acts as a nucleus for polymerization.

TLS Online TPP Program

#Id: 8319

#Unit 2. Cellular Organization

Nucleation phase 

Lag period (Time taken for nucleation) Rate-limiting Step.

TLS Online TPP Program

#Id: 8320

#Unit 2. Cellular Organization

The lag phase can be reduced or abolished entirely by adding premade nuclei.