Fig. 4.

Hh protein biogenesis and secretion: Hh is produced as a precursor protein ~ 45 kDa, transported to the ER, and post-translationally modified to generate a biologically active Hh protein. Hh pre-protein comprises a signal sequence, an N-terminal domain, and a C-terminal domain. Upon entrance into the secretory pathway, the N-terminal signal sequence is cleaved. Next, an autocatalytic reaction removes the C-terminal domain of Hh and catalyzes an intramolecular cholesteryl transfer reaction at the C-terminal. The final modification is completed by adding a palmitic acid moiety on a cysteine residue near the N-terminus by the acyltransferase skinny hedgehog. The dually modified Hh is secreted and released by the multi-pass transmembrane protein Disp 1 into the extracellular compartment. Disp facilitates the assembly of soluble, high molecular weight multimeric complexes. Hh protein is delivered to responsive cells via cytonemes and released from the plasma membrane via multiple possible mechanisms as a soluble multimer, a nodal vesicular parcel, or within lipoprotein particles or exovesicles