signaling molecules that are released and bind to receptors on the opposite membrane

short-lived, taken up quickly

ie. proteins, NT, peptide hormones

signaling molecules that cross plasma membranes to act on receptors within the cell

may persist in the bloodstream for hours or days

ie. steroids, retinoids, thyroid hormones

signaling molecules embedded in membrane that connect directly with receptors on other cells. they don't move!

important for neuronal development

channel-linked receptors

(ligand-gates ion channels)

have extracellular binding site for signal molecules. Binding activates intracellular enzymes which produce a product.

ie. receptors can be protein kinases that phosphorylate tyrosine residues

activated by cell-permeant molecules, which changes shape often exposing a DNA binding site.

leads to transcription and new mRNA and protein is made

signal molecule binds to receptor, G-protein (trimer of subunits α, β, and γ) binds to receptor, GTP binds to G-protein and activates it, α subunit breaks off and binds to effector protein, effector protein mediates responses

monomeric G-proteins

(small G-proteins)

protein activated by calcium

binds to and activated targets like protein kinases

• second messengers.
• cAMP and cGMP made from ATP and GTP by adenylyl cyclase and guanylyl cyclase.
• target protein kinase A and G.
• can also bind to ligand-gated ion channels (cyclic nucletide-gated channels) involved in smell and vision
• degraded by phosphodiesterases (turn them into AMP and GMP)

PIP₂ (membrane lipid) is cleaved by phospolipase C into DAG (diacylglycerol) which stays in the membrane and activates PKC (protein kinase C) and IP₃ which stays in the cytosol

enzymes degrade them back into PIP₂

molecule aiding in imaging

fluorescent dye that binds to calcium (causing it to fluoresce). allows us to see rising levels of intracellular calcium

enhanced green fluorescent protein (from jellyfish!)

tags protein

• kinases phosphorylate proteins and phosphatases remove phosphates
• act on serine or threonine residues (ser/thr kinases or phosphatases) and tyrosine residues (tyr kinases or phosphatases)
• regulated by second messengers or growth factors

• cAMP-dependent protein kinase 
• cAMP binds to PKA and activates it
• PKA unfolds and releases activated catalytic domains that phosphorylate substrates (serine and threonine residues)

• Ca²⁺/calmodulin-dependent protein kinase type II
• calcium/camodulin binds and activates CaMKII
• catalytic domain becomes active and can phosphorylate substrates like ion channels and serine/threonine residues

• protein kinase C
• calcium binds to it, binds to DAG in the membrane and catalytic domain becomes active and can phosphorylate substrates 

• act on tyrosine residues (important for cell growth and differentiation)
• receptors dimerize when signal molecule binds
• non-receptor tyrosine kinase is G-protein activated 

• mitogen-activated protein kinase
• part of kinase cascades (kinases activating other kinases that activate more kinases, etc)
• induces mitosis

upstream activating sequence

binds to DNA to activate transcription

• nerve growth factor
• protein required for differentiation and synaptic connections
• NGF binds to TrkA (receptor) which dimerizes
• the dimerized receptor phosphorylates itself and initiates ras pathway, IP₃ and DAG (phospholipase C), and other pathways

• long-term depression
• synaptic plasticity in PF synapses