Regulatory associated protein of FRAP, also designated Raptor, is a binding partner for mammalian target of rapamycin kinase (FRAP), and is essential for FRAP signalling in vivo. Raptor binding to FRAP is critical for FRAP-catalysed substrate phosphorylation of 4E-BP1. The raptor-FRAP complex is nutrient-sensitive and is important for a mechanism by which cells coordinate cell growth and size with changing environmental conditions. Raptor serves as a negative regulator of FRAP kinase activity under nutrient-deprived conditions and is an important component in the FRAP pathway. Raptor is highly expressed in skeletal muscle and to a lesser extent in brain, kidney, lung and placenta.