Biology 457 & 557 -  Week 10

intracellular signal transducers:

            the largest class of oncogenes are those whose proto-oncogenes encode proteins that act as

            one principal mitogenic signaling pathway in many cell types involves the

activation of

Ras proteins compose a large family of small guanine nucleotide-binding proteins

in order to activate Ras proteins, there is a binding between an

Grb2

Sos

this complex can activate Ras by promoting the conversion of Ras-GDP

the association of the Grb2-Sos complex with the receptor activates Sos

on exchanging GDP for GTP, Ras undergoes a conformational change

this cascade of signaling proteins conveys growth factor-initiated signals to the Raf protein

activated Mek then can phosphorylate another protein kinase known as Map kinase

unlike Raf and Mek, which are rather specific for their kinase substrates, Map

kinase phosphorylates a wide range of proteins

blocking the PTK/Ras/Map kinase mitogenic pathway at any level

normally, the Ras proteins can be inactivated by the hydrolysis of GTP to GDP

            this GTPase activity can be greatly enhanced by the interaction of Ras with

these include the proteins: p120-GAP, GAP-1, and neurofibromin

the large number of steps involved in joining the receptor PTK to nuclear events likely allows for amplification of the

how can this pathway turn oncogenic?

lastly, it has been determined that many human tumors have activated ras genes

these Ras proteins are actually pared down versions of so-called G proteins

            they are membrane-associated proteins that mediate signaling by

            upon sensing that a ligand has bound to the cell-surface receptor, G proteins

(specifically the Gs subunit) will bind

            normally, in unstimulated cells, Gs is bound to

            when a signal is sent (binding of ligand to receptor), there is a resultant

conformational change in the receptor’s cytoplasmic region

Gs-GTP can then interact with its target adenylate cyclase

            as a result, high levels of cyclicAMP are produced, which stimulates cAMP-

dependent kinase

however, formation of Gs-GTP is transient

            a mutation in the Gs protein eliminates the GTPase activity, thus causing the

            growth signal to be

            mutant forms of these Gs proteins have reduced ability to break down GTP and

this limits the self-deactivation function

            one mutation in the Gs protein actually produces the oncogenic potential:

                        substitute

                        this simple mutation is sufficient to change the

            another well-studied ex: c-src (identified in chicken Rous sarcoma virus)

                        the normal protein product is a nonreceptor protein-tyrosine kinase

                        these proteins are cytoplasmic or nuclear proteins

                        they function by partially binding to the plasma membrane

a signal can be passed from receptor to this

                        if phosphorylated, there is a great reduction in

                        if there is a deletion of this site, then there is

                        thought that Src plays a critical role in both

                        Src then becomes a frequent target for expression in human cancers

a third major pathway which is activated by surface receptor binding which transmits its signal through second messengers in the cytoplasm

            called the

            uses second messengers

            much like the ex. of G proteins in the cAMP pathway, a hormone or growth factor binds to a

but instead of using the Gs-GTP molecule to stimulate the next target, a Go-GTP molecule is

the active Go protein activates the enzyme

InsP₃ is released into the cytoplasm where it triggers the release of

this Ca²⁺, along with DAG, activate a family of other plasma-membrane protein kinases

have found that the cAMP and InsP₃/DAG pathways frequently interact

many oncogenes producing uncontrolled cell division encode the proteins involved in

finally, the downstream targets of these signaling pathways are proteins which are phosphorylated and directly or indirectly modify

a few more details on these signaling molecules in these pathways:

            there are regions w/in certain proteins that play a role in mediating protein-protein interactions

            SH2-containing proteins bind to specific phosphotyrosine residues

how is this done?

these proteins bind through a conserved polypeptide domain called

named because it has homology with a region in the cytosolic tyrosine kinase

each protein binds to a distinct sequence of amino acids surrounding a phosphotyrosine residue

            ex: the SH2 domain of the Src tyrosine kinase binds strongly

            it resembles the insertion of a two-pronged plug

other binding domains are not characterized

finally, the general structure of G protein-linked receptors themselves exhibit

            it was determined that all receptors of this type contain

            also called seven-spanning receptors

            the loop in the receptor between alpha helices 5 and 6 on the cytosolic side is important