Which statements are true? Explain why or why not.1 All second messengers are water-soluble and dif-fuse freely through the cytosol.2 In the regulation of molecular switches, proteinkinases and guanine nucleotide exchange factors (GEFs)always turn proteins on, whereas protein phosphatasesand GTPase-activating proteins (GAPs) always turn pro-teins off.3 Most intracellular signaling pathways providenumerous opportunities for amplifying the responses toextracellular signals.4 Binding of extracellular ligands to receptor tyro-sine kinases (RTKs) activates the intracellular catalyticdomain by propagating a conformational change acrossthe lipid bilayer through a single transmembrane α helix.5 Protein tyrosine phosphatases display exquisitespecificity for their substrates, unlike most serine/thre-onine protein phosphatases, which have rather broadspecificity.6 Even though plants and animals independentlyevolved multicellularity, they use virtually all the same sig-naling proteins and second messengers for cell–cell com-munication.
Which statements are true? Explain why or why not.
1 All second messengers are water-soluble and dif-
fuse freely through the cytosol.
2 In the regulation of molecular switches, protein
kinases and guanine
always turn proteins on, whereas protein phosphatases
and GTPase-activating proteins (GAPs) always turn pro-
teins off.
3 Most intracellular signaling pathways provide
numerous opportunities for amplifying the responses to
extracellular signals.
4 Binding of extracellular ligands to receptor tyro-
sine kinases (RTKs) activates the intracellular catalytic
domain by propagating a conformational change across
the lipid bilayer through a single transmembrane α helix.
5 Protein tyrosine phosphatases display exquisite
specificity for their substrates, unlike most serine/thre-
onine protein phosphatases, which have rather broad
specificity.
6 Even though plants and animals independently
evolved multicellularity, they use virtually all the same sig-
naling proteins and second messengers for cell–cell com-
munication.
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