Homo sapiens Tyrosine Kinase Domain of Fibroblast Growth Factor Receptor 1

Luke Kresslein '15 and Lillian Spetrino '15


I. Introduction

Fibroblast growth factor receptor 1 (FGFR1) is a tyrosine kinase protein that binds to a growth factor which plays an important role in cell growth, proliferation, differentiation, metabolism and angiogenesis. When fibroblast growth factor 1 (FGF1) in conjunction with cell surface-bound heparin sulfate proteoglycans bind to the extracellular domain of FGFR1, dimerization occurs. As a result of dimerization, specific tyrosine residues are autophosphorylated. Dimerization is essential for activation.

In this case, focus will be on the intracellular region of the FGFR1 (FGFR1K). This region houses the active site and is responsible for AMP-PCP binding as well as autophosphorylation sites. FGFR1K has been found to be inhibited by PD 173074, a tyrosine kinase inhibitor that is highly selective for FGFR1. It has been found to block angiogenesis induced by fibroblast growth factor with no toxicity. 

II. General Structure

FGFR1K is a bi-lobate intracellular protein tyrosine kinase (PTK) domain composed of a N-terminal lobe and a C-terminal lobe . The N-terminal lobe binds ATP and is composed of a curled beta sheet of five anti-parallel beta strands (sheet 1-sheet5) and alpha helix (helix C) . Contained in the N-terminal lobe are Trp-471 and Glu-472 which make up a WE sequence motif and provide structural stability in the N-terminal domain and establishes the boundry between the FGFR1K domain and the adjacent jutxamembrane domain (not shown). The intracellular C-terminal lobe performs substrate peptide binding and catalysis and is composed of two beta strands (beta sheet 7 and beta sheet 8 ) and seven alpha helices (helix D, helix E, helix F, helix EF, helix G, helix H, and helix I) ). The catalytic loop is found in the C-terminal lobe and is between helix E and beta sheet 7.

III. Active-Site and AMP-PCP Binding

The catalytic loop of the protein kinase lies between helix E and beta sheet 7 with Asp-623 acting as the catalytic base in the phosphotransfer reaction. The active sites of the two FGFR1K molecules in the asymmetric unit are unoccupied. In fact, Tyr-653 and Tyr-654 are observed to point away from the active site .

The side chain of Tyr-653 is in van der Waals contact with Val-664, Leu-672 and Phe-710, and is hydrogen bonded to a backbone carbonyl oxygen (Leu-672O) . Tyr-654 is more solvent exposed than Tyr-653, and only has van der Waals contact with Val-706 . When AMP-PCP binds, there are no significant changes in the structure of FGFR1K and biding of AMP-PCP does not promote lobe closure on its own, nor does binding result in any noticeable changes in the conformations of the activation and nucleotide-binding loops.

The adenine ring of AMP-PCP hydrogen bonds between N1 and the amide nitrogen of Ala-564 and between N6 and the carbonyl oxygen of Glu-562 On either side of the adenine ring is Leu-484 and Val-492 on the N-terminal lobe and Leu-630 on the C-terminal lobe on the other side of the ring.

IV. Autophosphorylation and Dimeric Forms

Due to the special positioning of the sites relative to the active site, autophosphorylation occurs by a trans mechanism, even though Tyr-653 and Tyr-654 are near enough to the active site to be autophosphorylated in cis. There are two tyrosine autophosphorylation sites in the activation loop (Tyr-653 and Tyr-654) in addition to four others. These four are Tyr-463, two in the kinase insert between helix D and helix E (Tyr-583 and Tyr-585), that were not found in the crystal, and one situated in helix H of the core of the C-terminal lobe (Tyr-730) . Because Tyr-730, Met732 and Met-733 are buried , helix H must unfold.

V. Inhibition by PD173074

PD 173074 binds in the ATP-binding cleft of FGFR1K, which lies between the two lobes of the kinase. The pyrido[2,3-d]pyrimidine ring system occupies the same position as the ATP adenine. Hydrophobic residues that line the pyrido[2,3-d]pyrimidine/adenine pocket include Leu-484 , Ala-512 Tyr-563 , Ala-564 and Leu-630 . PD 173074 makes two hydrogen bones with main-chain atoms of FGFR1K in the segment that connects the two kinase lobes, one between N-3 of the pyrimidine ring and the amide nitrogen of Ala-564 and the other between the nitrogen of the butylamino group and the carbonyl oxyen of Ala-564. There is also a hydrogen bond between one of the dimethoxy groups of PD 173074 and the amide nitrogen of Asp-641. A water mediated hydrogen bond occurs between the urea group attached to the pyrido[2,3-d]pyrimidine at the 7-position that makes a hydrogen bond with N-8 of the ring system (not cystalized). This is also hydrogen bonded to a water molecule which is in turn hydrogen bonded to Lys-514 and Asp-641 .

VI. References

Mohammadi M, Schlessinger J and Hubbard SR (1996) Structure of the FGF receptor tyrosine kinase domain reveals a novel autoinhibitory mechanism. Cell 86: 577-587.

Mohammadi M, Froum S and Hamby J (1998) Crystal structure of an angiogenesis inhibitor bound to the FGF receptor tyrosine kinase domain. The EMBO Journal 17: 5896-5904.

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