Smyd3 open & closed lock mechanism for substrate recruitment: the hinge motion of C-terminal domain inferred from μ-second Molecular Dynamics simulations

Background The human lysine methyltransferase Smyd3, a member of the SET and MYND domain containing protein family, harbors methylation activity on both histone and non-histone targets in a tightly regulated manner. The mechanism of how Smyd3 dynamically regulates substrate recognition is still not fully unveiled. Methods Here, we employed molecular dynamics simulations on full length human Smyd3, performed to a total of 1.2 μ-second, in the presence (holo) and absence (apo) of the S-Adenosyl methionine (AdoMet) cofactor. The dynamical features of Smyd3 in apo and holo states have been examined and compared via examining geometrical and electrostatic properties. Results The results show a distinct dynamics of the C-terminal domain (CTD) in the two states. In the apo state, the CTD undergoes a large hinge like motion and samples more opened configurations, thus acting like a loosened clamp and resulting in expanded substrate binding crevice. In the holo state, the CTD exhibits a restricted motion while the overall structure remains compact, mimicking a closed clamp. This leads to a localized increase in the negative potential at the substrate binding cleft. Further, solvent accessibility of critical residues at the target lysine access channel, important for methylation activity, is increased. Conclusions We postulate that AdoMet cofactor acts like a key and locks Smyd3 in a closed conformation. In effect, the cofactor binding restricts the elasticity of the CTD, presenting a compact substrate binding cleft with high negative potential, which may have implications on substrate recruitment via long range electrostatics.