Worm Breeder's Gazette 13(4): 80 (October 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have completed the sequence of the degenerin gene deg-1 ,which encodes a putative channel subunit. The degenerin family includes MEC-4 , MEC-10 ,and the three subunits of the mammalian epithelial sodium channel. All degenerins contain two hydrophobic domains predicted to span the membrane. LacZ fusion experiments indicate that the region between the hydrophobic domains is extracellular, and that the N- and C-termini are intracellular. Dominant gain-of-function mutations in deg-1 , mec-4 ,and mec-10 cause neuronal degeneration (the Deg phenotype). The touch receptor neurons die in mec-4 and mec-10 mutants, and a group of interneurons and sensory neurons die in deg-1 mutants. All dominant Deg mutations substitute a large amino acid for an alanine situated in the second hydrophobic domain, at the predicted pore of the channel. The position of these mutations is consistent with the hypothesis that they alter a residue of the pore to hyperactivate the channel. The hyperactivity of the channel would eventually lead to cell death. deg-1 ( u506 )is a recessive gain-of-function mutation that causes degeneration of at least some of the same neurons as the dominant deg-1 ( u38 ).The deg-1 ( u506 )mutation substitutes Thr for Ala at residue 393 in a 22 amino acid extracellular region that is found in the C. elegans degenerins but is absent in the mammalian proteins. An equivalent mutation of mec-4 (A385T) as well as a deletion of part of this domain (codons 380-389) result in the death of the touch cells. Thus, this area of the C. elegans degenerins appears to be important in gating the channel. Large amino acids at MEC-4 residue 385 (Thr, Leu, and Lys) also result in touch receptor degeneration, but small amino acids (Ala, Gly, and Cys) do not, suggesting that steric constraints at this residue are important for the function of this region. We have identified three deg-1 mutations ( u512 , u558 ,and u679 )that are dominant suppressors of the recessive u506 and of the dominant u38 Deg mutations. All three suppressor mutations substitute large residues for small residues in the predicted lining of the channel pore. The larger residues could be blocking the pore, and thus prevent the currents that would otherwise lead to cell degeneration. The trans-dominant suppressions suggest that deg-1 products interact, i.e. that the channel contains multiple copies of DEG-1 .This may explain why u38 is a dominant mutation while u506 is a recessive mutation: a defect on the pore domain of one subunit may be sufficient to force the multimeric channel open, but removal of only one of several inactivation domains would still allow the remaining domains to inactivate the channel. Analogous models could explain other recessive gain-of-function mutations. The occurrence of this putative inactivation domain in the C. elegans degenerins, but not in the mammalian channel subunits, may explain the different properties of the resulting channels. The rat epithelial channel (involved in sodium reabsorption in the kidney, colon, and lung) is constitutively active, with long open times. In contrast, the C. elegans channels are hypothesized to be gating rapidly and to be closed most of the time. The inactivation domain may make this rapid gating possible. We think it likely that genes encoding degenerin-like proteins with inactivation domains remain to be found in mammals, and these may, like those in C. elegans, underlie mechanosensory transduction.