Tyrosine Kinases

Tyrosine kinases

subset of kinases important for growth, cell division, and metabolism.

c-Src

The first proto-oncogene and an example of a tyrosine kinase.

Insulin receptor structure

heterodimer (a2b2) stabilised by disulfide bonds

where on the insulin receptor does insulin bind and what happens?

binds alpha2 subunits which transmit signal to beta2 → TK activation

Auto-phosphorylation

The process where a receptor phosphorylates its own tyrosine residues after ligand binding

Signalling complex

form when phosphorylated Tyr residues recruit and phosphorylate signalling molecules like IRS-1.

Dimerisation

pairing of receptors when ligand binds → activation loop moved

The insulin receptor is already a heterodimer. What happens instead of dimerisation when insulin binds?

Alpha subunits intertwine bringing the beta subunits closer together

How has the conformational change of insulin receptors been observed?

receptor incorporated into nanodisc. High res cryoelectron microscopy used to look at structural changes when insulin added

Trans autophosphorylation

receptor chain A phosphorylates specific tyrosines in receptor chain b. Phosphorylated tyrosines act as docking sites for signalling molecules

SH2 domains

Domains which bind phospho-tyrosine residues. Encoded by single exon

features found in all SH2 domains

pocket which P-Tyr fits

Specificity in SH2 binding

each SH2 domain has different specificity pocket

Why can the same receptor have different functions in different cell types?

different downstream effectors - signalling complex is different

Leptin

hormone produced by fat cells which binds cytoplasmic TK JAK2. Used in mouse obesity models

What happens after leptin receptor homodimerises and autophosphorylates?

JAK2 recruited. STAT3 phosphorylated and dimerises. Dimer enters nucleus → transcription

JAK in breast cancer stem cells (BCSC)

regulates BCSC lipid metabolism via STAT3 pathway. Promotes breast cancer and chemoresistance (Wang et al, 2018)

JAK inhibition in BCSC

reduces fatty acid oxidase activity and decreases BCSC self renewal

PLC-γ activation (non tyrosine receptor route)

• active GPCR alpha subunit activates phospholipase C

• phospholipase C cleaves PIP2 → IP3 and DAG

• IP3 binds ER Ca channel receptor → Ca release

• DAG and calcium activate PKC

PLC gamma activation by TK receptors

PLC gamma recruited to receptor via SH2 domain and is phosphorylated by TK receptor domain

Ras

G protein

Ras-GTP

activates signalling pathway

Ras-GDP

inactive form

control of Ras-GTP:Ras-GDP

controlled by relative activity of Ras-GAP and SOS proteins. Key control is recruitment of these proteins to receptors by SH2 domains

MAPK cascade

• RTK phosphorylates Shc

• Shc recruits Grb2, SOS and Ras

• Ras GTP activates Raf1 kinase (MAPKKK)

• Raf1 kinase phosphorylates MEK 1 or 2 (MAPKK)

• MEK 1 or 2 phosphorylates ERK 1 or 2 (MAPK)

Ras negative feedback

production of ERK 1/2 inhibits upstream signalling components Raf1 and SOS

Ras-GAP

forms Ras GDP stopping the pathway

How are JNK and p38 MAPK pathways activated?

mainly via environmental stresses and proinflammatory stimuli