Protein phosphatases
Protein phosphatase acting as a reset button for kinases this action is either a) inactivating or b) activating
like for protein kinases, a distinction is made between tyrosine and threonine/serine phosphatases As there is always an exception to the rule: certain phosphatases recognize both phosphotyrosine and phosphothreonine/serine residues these are the dual specificity phosphatases
serine threonine tyrosine serine threonine tyrosine { { {
The depth of the catalytic pocket is one of the determinants of phosphoresidue specificity of the protein phosphatase
Serine / threonine phosphatases
Mechanism of action and highly conserved (signature) sequences of serine / threonine protein phosphatases. The catalytic subunits have broad specificity
Specificity of serine/threonine phosphatases is largely determined by association of regulatory subunits that affect subcellular localisation as well as substrate specificity
The regulatory subunit MYPT1 turns the broad-specificity PP1C (catalytic subunit) into a myosin regulatory-light chain-specific phosphatase - R= arginine & K= lysine are positively charged residues that fit the negatively charged acidic groove - Dephosphorylation of myosin regulatory light chain causes smooth muscle cell relaxation (no hydrolysis of ATP)
Role of the phosphatase calcineurin in antigen-mediated T-cell activation (followed by clonal expansion (adaptive immunity))
the activated T-cell receptor induces a Ca 2+ signal that binds calcineurin and activates the catalytic subunit (CnA) (PP2B), leading to dephosphorylation and nuclear translocation of the transcription factor NFAT NFAT=nuclear factor of activated T-cells
action of protein phosphatase
tyrosine protein phosphates (PTP)
two step mechanism of action of protein tyrosine phosphatases
Tyrosine phosphatase as an inactivator of insulin signalling: tyrosine phosphatase 1B (PTP1B) dephosphorylates the insulin receptor
Certain tyrosine phosphatases have tumour suppressor functions (by opposing to deragulated tyrosine protein kinases)
tyrosine phosphatase as an activator of Src: dephosphorylation of the C-terminal tyrosine phosphate by CD45
the SH2 domain bound to the C- terminal phosphotyrosine keeps the kinase in an incompetent conformation dephosphorylation removes this constraint and is followed by autophosphorylatio n. The kinase is now fully competent
PTEN qualifies by its structure and its sequence as a tyrosine phosphatase but it prefers inositol lipids and acts as a inositol lipid phosphatase (important tumour suppressor role)
PTEN removes the phosphates that have been added by PI 3- kinase PI 3-kinase is activated by growth factors (including insulin) and plays an important role in growth regulation and cell survival this explains why loss of PTEN is involved in cell transformation (cancer development)