Three key residues underlie the differential affinity of the TGFbeta isoforms for the TGFbeta type II receptor

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• Transforming growth factor-beta (TGFbeta) isoforms 1, 2, and 3 are critical signaling molecules with distinct functions in development, tumor suppression, and wound healing.
• These isoforms bind to TGFbeta type I and type II receptors (TbetaRI and TbetaRII) to transduce signals, but TGFbeta2 exhibits weaker binding to TbetaRII and relies on the co-receptor betaglycan.
• Understanding the molecular basis for these differential binding affinities is crucial for elucidating TGFbeta signaling pathways.

Purpose of the Study:

• To investigate the physicochemical basis for the differential binding affinities of TGFbeta isoforms to the TGFbeta receptor type II (TbetaRII).
• To identify specific amino acid residues in TbetaRII that contribute to ligand binding affinity and equilibrium stability for TGFbeta isoforms.
• To determine how specific amino acid substitutions in TGFbeta2 affect its binding affinity to TbetaRII.

Main Methods:

• Generation of single amino acid variants of TbetaRII based on the crystal structure of the TbetaRII:TGFbeta3 complex.
• Assays to measure TGFbeta isoform binding affinity to wild-type and variant TbetaRII.
• Analysis of equilibrium stability of TbetaRII variants in complex with TGFbeta isoforms.

Main Results:

• Specific TbetaRII residues (Ile53, Glu119, Asp32, Glu55, and Glu75) were identified as significant contributors to TGFbeta3 binding affinity.
• These contacts account for the weaker binding affinity of TbetaRII to TGFbeta2, attributed to conservative substitutions at ligand residues Lys25, Ile92, and Lys94 in TGFbeta2 compared to TGFbeta1 and TGFbeta3.
• A TGFbeta2 variant with residues mutated to match TGFbeta1 and TGFbeta3 exhibited comparable receptor binding affinities, confirming the role of these three residues.

Conclusions:

• Three specific amino acid residues in TGFbeta2 are responsible for its lower binding affinity to TbetaRII compared to TGFbeta1 and TGFbeta3.
• These identified residues and their interactions provide a molecular explanation for the differential binding of TGFbeta isoforms to their receptor.
• All TGFbeta isoforms likely assemble the TGFbeta signaling receptor complex in a similar overall manner, despite variations in initial binding affinity.