The biosynthesis of gibberellins is under strict genetic, developmental, and environmental control, and numerous gibberellin-deficient mutants have been isolated. 

Mendel’s tall/dwarf alleles in peas are a famous example. Such mutants have been useful in elucidating the complex pathways of gibberellin biosynthesis.

TLS Online TPP Program

#Id: 9632

#Unit 4. Cell Communication and Cell Signaling

Ras has an intrinsically slower rate of GTP hydrolysis, so Ras∙GTP requires the assistance of another protein, GAP.

TLS Online TPP Program

#Id: 9633

#Unit 4. Cell Communication and Cell Signaling

Binding of a GAP to Ras⋅GTP accelerates the intrinsic GTPase activity of Ras by more than a hundredfold

TLS Online TPP Program

#Id: 9634

#Unit 4. Cell Communication and Cell Signaling

The GAP for RAS (called a RAS-GAP) works by inserting one of its arginine side chains into the Ras active site, thus stabilizing an intermediate in the

hydrolysis reaction.

TLS Online TPP Program

#Id: 9635

#Unit 4. Cell Communication and Cell Signaling

Constitutively active Ras proteins (RasD) 

Replacement of the normal glycine 12 with any other amino acid (except proline) blocks the functional binding of GAP proteins and in essence “locks” Ras in the active GTP-bound state.

TLS Online TPP Program

#Id: 9636

#Unit 4. Cell Communication and Cell Signaling

Activated RTKs and cytokine receptors to activate Ras, by two cytosolic proteins — GRB2 and Sos

TLS Online TPP Program

#Id: 9637

#Unit 4. Cell Communication and Cell Signaling

GRB2 is an adapter protein, serves as a link, or scaffold, between the activated receptor and Sos