Allolactose binds to the Lac repressor and triggers a change in the shape (conformation) of that protein. 

TLS Online TPP Program

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#Unit 4. Cell Communication and Cell Signaling

Other EGF family receptors HER2, HER3, and HER4 do not undergo internalization after exposure to ligand

TLS Online TPP Program

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#Unit 4. Cell Communication and Cell Signaling

Monomeric Ras protein 

(∼170 amino acids) belongs to the GTPase superfamily of

intracellular switch proteins

TLS Online TPP Program

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#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

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#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

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#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

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#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.