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.


Jaime Garcia-Anoveros Marty Chalfie Department of Biological Sciences, Columbia University, New York, NY 10027.

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.