Defining Noncovalent Ubiquitin Homodimer Interfacial Interactions through Comparisons with Covalently Linked Diubiquitin.

Covalently linked diubiquitin (diUbq) is known to adopt specific interfacial interactions owing to steric hindrance induced by the covalent tether. K48-linked diUbq preferentially forms hydrophobic interfacial interactions between the two I44 faces under physiological conditions, whereas K63-linked diUbq preferentially forms electrostatic interfacial interactions. Here, we show using collision-induced unfolding ion mobility-mass spectrometry that the recently reported noncovalent dimer of ubiquitin exhibits structural preferences and interfacial interactions that are most similar to that of K48-linked diUbq.