Combining direct methods with isomorphous replacement or anomalous scattering data. VIII. Phasing experimental SIR data with the replacing atoms in a centrosymmetric arrangement

The LisH motif of muskelin is crucial for oligomerization and governs intracellular localization

Neurons regulate the number of surface receptors by balancing the transport to and from the plasma membrane to adjust their signaling properties. The protein muskelin was recently identified as a key factor guiding the transport of α1 subunit-containing GABAA receptors. Here we present the crystal structure of muskelin, comprising its N-terminal discoidin domain and Lis1-homology (LisH) motif. The molecule crystallized as a dimer with the LisH motif exclusively mediating oligomerization. Our subsequent biochemical analyses confirmed that the LisH motif acts as a dimerization element in muskelin. Together with an intermolecular head-to-tail interaction, the LisH-dependent dimerization is required to assemble a muskelin tetramer. Intriguingly, our cellular studies revealed that the loss of this dimerization results in a complete redistribution of muskelin from the cytoplasm to the nucleus and impairs muskelin's function in GABAA receptor transport. These studies demonstrate that the LisH-dependent dimerization is a crucial factor for muskelin function.

Applications of direct methods in protein crystallography for dealing with diffraction data down to 5 Å resolution

Apart from solving the heavy-atom substructure in proteins and ab initio phasing of protein diffraction data at atomic resolution, direct methods have also been successfully combined with other protein crystallographic methods in dealing with diffraction data far below atomic resolution, leading to significantly improved results. In this respect, direct methods provide phase constraints in reciprocal space within a dual-space iterative framework rather than solve the phase problem independently. Applications of this type of direct methods to difficult SAD phasing, model completion and low-resolution phase extension will be described in detail.