USB1 sequence alignment and structural homology. (A) Structure-based sequence alignment of human USB1 homologues from various species. Shading intensity indicates the degree of amino acid identity. Secondary structure elements for H sapiens USB1 are shown below the alignment with the terminal lobe colored magenta, the transit lobe cyan. The invariant H-x-S motifs in the active site cleft are indicated in red. GenBank accession numbers: H sapiens (NP_078874.2), D rerio (NP_001003460.1), S pombe (NP_593641.2), S cerevisiae (NP_013233.1), D melanogaster (NP_649911.1), and C glabrata (CAG62512.1). The secondary structure elements of USB1 were calculated from the crystal structure using DSSP⁴⁴  and the alignment generated using T-coffee and Jalview.^45,46 (B) Superposition of the active site of human USB1 (PDB 4H7W, Cα chain, light blue; side-chains, blue) and rat AKAP18δ (PDB 2VFY, Cα chain, light red; side-chains, red). The catalytic residues are shown as ball-and-stick models. Human USB1 is numbered in blue, AKAP18δ in red.