ROLE OF DIMERIZATION OF THE MEMBRANE-ASSOCIATED GROWTH-FACTOR KIT-LIGAND IN JUXTACRINE SIGNALING - THE SL(17H) MUTATION AFFECTS DIMERIZATION AND STABILITY-PHENOTYPES IN HEMATOPOIESIS

The Kit ligand (KL)/Kit receptor pair functions in hematopoiesis, game togenesis, and melanogenesis. KL is encoded at the murine steel (SI) l ocus and encodes a membrane growth factor which may be proteolytically processed to produce soluble KL. The membrane-associated form of KL i s critical in mediating Kit function in vivo. Evidence for a role of c ytoplasmic domain sequences of KL comes from the Sl(17H) mutation, a s plice site mutation that replaces the cytoplasmic domain with extraneo us amino acids. Using deletion mutants and the Sl(17H) allele, we have investigated the role of the cytoplasmic domain sequences of KL in bi osynthetic processing and cell, surface presentation. The normal KL pr otein products are processed for cell surface expression, where they f orm dimers. Both Sl(17H) and the cytoplasmic deletion mutants of KL we re processed to the cell surface; however, the rate of transport and p rotein stability were affected by the mutations. Deletion of cytoplasm ic domain sequences of KL did not affect dimerization of KL. In contra st, dimerization of the Sl(17H) protein was reduced substantially. In addition, we have characterized the hematopoietic cell compartment in Sl(17H) mutant mice. The Sl(17H) mutation has only minor effects on he matopoiesis. Tissue and peritoneal mast cell numbers were reduced in m utant mice as well as in myeloid progenitors. Interestingly, long-term bone marrow cultures from Sl(17H) mice did not sustain the long-term production of hematopoietic cells. In addition, homing of normal hemat opoietic progenitors to the spleen of irradiated Sl(17H)/Sl(17H) recip ient mice was diminished ill transplantation experiments, providing ev idence for a role of Kit in homing or lodging. These results demonstra te that the membrane forms of KL exist as homodimers on the cell surfa ce and that dimerization may play an important role in KL/Kit-mediated juxtacrine signaling.