The terrifying & unsolved mystery of genetic suppression by ADM-2

Loss of ADM-2/meltrin, a conserved matrix metalloprotease, suppresses molting defects of NIMA-kinase mutants of C. elegans

Mutations in the C. elegans NIMA-related kinase family members, nekl-2 and nekl-3, lead to defects in the ability of larvae to shed their old cuticles during molting cycles. In a screen for suppressors of nekl-2; nekl-3 molting defects, we identified two independent mutations affecting ADM-2, the ortholog of human ADAM9/12/19 and the sole meltrin-class matrix metalloprotease in C. elegans. Notably, it is loss of function in adm-2, including CRISPR-generated null alleles, that facilitate separation of the old and new cuticles in nekl-2; nekl-3 mutants. Although loss of function in several proteases has been reported to cause molting defects, no proteases have been previously reported to suppress a molting defect. This suggests that ADM-2 may promote molting through a mechanism other than cuticle degradation.
By carrying out a structure function analysis of adm-2 using CRISPR mutagenesis, we identified several critical domains required for ADM-2 function. We find that disruption of the extracellular Zn-binding protease active site is sufficient to yield high levels of suppression, demonstrating the importance of this catalytic activity. Moreover, we find a critical role for the cytoplasmic C-terminus of ADM-2, including one of the three SH3-binding domains. This domain in humans has previously been shown to interact with sorting nexin-9 (SNX9), providing a potential link between ADM-2 and the NEKLs, which function in clathrin mediated endocytosis.
Most surprisingly, we find a major function for a putative nuclear-localization signal in the C-terminus of ADM-2, as well as a predicted furin cleavage site located just inside the transmembrane domain. These latter findings suggest that the cytoplasmic domain of ADM-2 may be cleaved and enter the nucleus. Consistent with this possibility, a ADM-2::GFP reporter is observed on plasma membranes and in the cytoplasm and nuclei of ectodermal cell types, including hyp7, which is the cellular focus of NEKL-2 and NEKL-3 activity. Notably, several recent studies have suggested functions for the C-terminal domains of ADAM-family proteases, however, little is known about their potential roles in signaling.