The steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domain is a protein module of approximately 210 residues that binds lipids, including sterols. The START domain consists of an alpha/beta helix-grip fold. Fifteen mammalian proteins, STARD1-STARD15, possess a START domain []. This entry includes StARD5 and StARD6.StAR-related lipid transfer protein 5 (StARD5) contains a START domain that binds lipids, including sterols []. StARD5 primary binds bile acids, and may be involved in the transport of bile acids rather than cholesterol [].StARD6 can bind cholesterol. It has cholesterol-transfer activity and StAR-like activity. STARD6 is restricted to the testes and is expressed during spermatogenesis in spermatids but not in steroidogenic cells [, ].
