Falcon (Edison Scientific Literature) deep research report: mouse F2rl2 (PAR3, UniProt O08675)
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Falcon synthesizes that mouse PAR3 is a rhodopsin-family 7TM GPCR activated by proteolytic N-terminal cleavage and tethered-ligand exposure, consistent with UniProt O08675/PAR3 identity.
"The target is **mouse F2rl2 (Par3)** encoding **proteinase-activated receptor 3 (PAR3)**, a rhodopsin-like **7-transmembrane GPCR** activated by proteolysis"
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In mouse platelets the dominant, experimentally supported role of PAR3 is to cofactor PAR4 activation rather than to act as an autonomous signaling receptor; PAR3 does not itself transduce signal upon thrombin cleavage but enables PAR4 signaling.
"PAR3 itself is stated to **not mediate signal transduction upon thrombin cleavage**, instead acting as a **cofactor enabling PAR4 signaling**"
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A consistent theme is that PAR3 acts both as a thrombin-binding cofactor/modulator for other PARs (especially PAR4 on mouse platelets) and as a signaling receptor in certain non-platelet contexts where direct downstream signaling has been demonstrated (e.g., islets/beta cells).
"A consistent theme is that PAR3 can act (i) as a **thrombin-binding cofactor/modulator** for other PARs (especially PAR4 on mouse platelets), and (ii) as a **signaling receptor** in certain non-platelet contexts where direct downstream signaling has been experimentally demonstrated (e.g., islets/β cells)"
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Mouse PAR3 has a hirudin-like thrombin-binding sequence that binds thrombin exosite I and is cleaved at Lys38/Thr39 to expose a TFRGAP-initiated tethered ligand.
"Mouse PAR3 is reported to have a **hirudin-like thrombin-binding sequence** that can bind thrombin’s **exosite I**, and it is cleaved at **Lys38/Thr39**, exposing a tethered ligand beginning with **TFRGAP…**"
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PAR3/PAR4 heterodimerization recruits thrombin to facilitate cleavage of adjacent PAR4, and dimerization is described as essential for this amplification of PAR4 cleavage.
"A mechanistic model is that PAR3’s hirudin-like/exosite-I thrombin binding recruits thrombin to facilitate cleavage of adjacent PAR4; **dimerization is described as essential** for this amplification of PAR4 cleavage"
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In mouse islets/beta cells, PAR3 activation signals via Gq to PLC and IP3/DAG, with Ca2+ release from intracellular stores and PKC involvement.
"Pharmacology and Ca2+ imaging support signaling via **Gq → PLC → IP3/DAG**, with **Ca2+ release from intracellular stores** and involvement of **PKC**"
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A PAR3 activating peptide at 20 microM increased insulin secretion ~90% at basal glucose and ~130% at high glucose and increased beta-cell exocytosis ~70%.
"a PAR3 activating peptide at **20 μM** robustly increased insulin secretion (reported ~**90%** increase at basal glucose and ~**130%** at high glucose) and increased β-cell exocytosis (~**70%**)"
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Species difference in platelet PAR receptor usage - mouse platelets express a PAR3/PAR4 complex while human platelets express PAR1 and PAR4, so Par1-/- mouse phenotypes should not be read as platelet thrombin hyporesponsiveness.
"mouse platelets express a **PAR3/PAR4 complex**, while human platelets express **PAR1 and PAR4**, and thus phenotypes in Par1−/− mice should not be assumed to reflect platelet hyporesponsiveness to thrombin"
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An inflammation-focused PAR review summarizes that mouse PAR3 maintained thrombin recruitment activity but lost receptor function, reinforcing the thrombin-recruiting cofactor concept in mice.
"mouse PAR3 maintained thrombin recruitment activity but “lost receptor function,”"
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PARs including PAR3 are cell-surface transmembrane receptors whose activation requires extracellular proteolysis; after activation they internalize and can form endosomal beta-arrestin signaling complexes.
"PARs, including PAR3, are **cell-surface transmembrane receptors** whose activation requires extracellular proteolysis; after activation they undergo **internalization** and can form endosomal signaling complexes via β-arrestin"