Deleting the conserved hexapeptide in DLK-1L also completely abolished rescuing activity ( Figure 2, juEx4098). Together, these results identify the conserved C-terminal hexapeptide as critical for DLK-1L function. To determine how DLK-1S interacts with DLK-1L and how the C-terminal hexapeptide regulates their small molecule library screening interaction, we next performed protein interaction studies using yeast two-hybrid assays. We found that full-length DLK-1L interacted with itself and also with DLK-1S (Figure 3B). Removal of the LZ domain in DLK-1L or DLK-1S eliminated these interactions. Unexpectedly, despite containing the LZ domain, DLK-1S did not show interaction with itself, suggesting that the LZ domain is not

sufficient for DLK-1 dimerization or oligomerization. To test the role of the C-terminal hexapeptide learn more SDGLSD in the interactions between DLK-1 isoforms, we deleted it from DLK-1L. We found that a DLK-1L construct lacking the hexapeptide failed to show any homomeric interaction and, instead, displayed an enhanced heteromeric interaction with DLK-1S (Figure 3C). These

results suggest that the C-terminal hexapeptide plays a critical regulatory role in DLK-1 isoform-specific interactions. Since the C-terminal aa 856–881 region can endow a truncated DLK-1(kinase+LZ) with complete function (Figure 2, juEx3588), we tested whether this domain might interact with the kinase domain. In yeast two-hybrid assays, we observed that the aa 850–881 region interacted with the kinase domain of DLK-1 ( Figure 3E). The hexapeptide SDGLSD contains two potential phosphorylation sites (Ser 874 and Ser878, Figure 3A). We addressed whether these serines were sites of regulation by generating phosphomimetic and nonphosphorylatable forms of the hexapeptide. We found that full-length DLK-1L containing phosphomimetic (S874E, S878E) hexapeptide showed stronger binding to itself ( Figure 3C). Conversely, full-length DLK-1L

containing nonphosphorylatable (S874A, S878A) hexapeptide showed an enhanced interaction with DLK-1S ( Figure 3C). The phosphomimetic C-terminal aa 850–881 region also showed stronger binding to the kinase domain of DLK-1 ( Figure 3E). The C-terminal domain alone did not interact with itself in yeast two-hybrid assays (data not shown), although a region 3-mercaptopyruvate sulfurtransferase of 209 amino acids between LZ domain and the hexapeptide was necessary for DLK-1L to interact with DLK-1S ( Figure 3D). Taken together, the results from yeast two-hybrid interaction assays suggest that phosphorylation of the DLK-1L hexapeptide could regulate the balance between active DLK-1L homomers and inactive DLK-1L/S heteromers. To address the in vivo importance of DLK-1 C-terminal hexapeptide phosphorylation, we turned to transgenic expression. DLK-1L with a nonphosphorylatable hexapeptide (S874A, S878A) was expressed normally (Figure S4) but lacked rescuing activity (Figure 4A, juEx4708).