Long range signaling of Hh depends on a number of additional factors that promote the release and spread of Hh-Np ligand. Dispatched2 (DISP2) is a 12-pass transmembrane protein that was identified as being required for release and long-range signaling by Hh-Np (Burke et al, 1999; Ma et al, 2002; Nakano et al, 2004). In mammals, DISP2 has been shown to bind directly to the cholesterol moiety of Hh-Np through its first extracellular region, and this interaction is required for secretion and long-range signaling. Mutants of DISP2 that are inactive in promoting Hh long-range signaling show tighter binding to Hh-Np, suggesting that these mutants interfere with the release of Hh ligand from the plasma membrane (Tukachinsky et al, 2012). Although this pathway shows DISP2 and Hh-Np interacting at the plasma membrane, it is possible that the interaction occurs earlier in the secretory pathway and that DISP2 escorts Hh-Np to the cell surface.
In addition to binding to DISP2, the cholesterol moiety of Hh-Np is also bound by the secreted protein SCUBE2 (Tukachinsky et al, 2012; Creanga et al, 2012). Like DISP2, SCUBE2 is required for long-range signaling by Hh-Np (Johnson et al, 2012; Tukachinsky et al, 2012; Creanga et al, 2012). DISP2 and SCUBE2 recognize distinct portions of the cholesterol group on Hh-Np, suggesting a possible hand-off mechanism at the plasma membrane that allows the release of Hh from the secreting cell (Tukachinsky et al, 2012; Creanga et al, 2012).
