Transmembrane kit ligand cleavage does not require a signal in the cytoplasmic domain and occurs at a site dependent on spacing from the membrane

The kit ligand (KL) is one of several growth factors that are active as transmembrane molecules and can also be proteolytically cleaved to yield soluble forms. We have investigated the signals and structural determinants that control the cleavage of KL. Presentation at the membrane appears to be critical, because no cleavage occurs in variants that lack a transmembrane domain. Signals in the cytoplasmic domain do not seem to be required, because cleavage was not blocked by removal of the C-terminal valine residue, deletion of the whole cytoplasmic tail, or the replacement of the cytoplasmic tail that occurs in the Sl(17H) mutation. KL thus appears to differ from transforming growth factor-α, which apparently requires a C-terminal valine as a signal for cleavage. Although proteolysis must be tightly restricted to the correct cell surface proteins and sites within each protein, cleavage of KL does not seem to be determined entirely by a requirement for a specific substrate sequence. However, the effects of deletion or insertion variants of KL suggest that cleavage may be limited to sites within a specific range of distances from the cell membrane.