The crystal structure of the yeast histidine phosphotransferase protein Ypd1. The four helices shown in yellow comprise the conserved four-helix bundle typical of HPt domains; the helices shown in red are insertions specific to Ypd1. The histidine phosphorylation site is shown in green. Compare this structure to 4fmt.png, depicting a dimeric Hpt. Rendered using PyMol from PDB ID 1C02.