The activity of the
mec-4 gene is normally required for the function of the six touch receptor neurons. Three dominant
mec-4 mutations result in the swelling and degeneration of these neurons. We are interested in describing in molecular detail how the degeneration of a neuron can occur. Our working hypothesis is that
mec-4 might act directly in the membrane to influence cell permeability, perhaps by acting as a component of a transport system or mechanosensory channel. Previously, we sequenced a partial
mec-4 cDNA and reported that the deduced protein sequence (497 amino acids) included two cys-rich domains, a putative membrane spanning domain, and a positively charged C-terminus. To determine likely coding regions in the S' end of the gene we have compared the genomic sequences of the
mec-4 genes of C. elegans and C. briggsiae. All degeneration-inducing
mec-4 mutations result in amino acid substitutions for an alanine residue at position 442. Because any amino acid with a side chain larger than that of Ala results in degeneration, steric hindrance is thought to play a role in the degeneration mechanism. Ala-442 falls within a domain that is highly conserved between
mec-4 and deg-l, another gene that can mutate to cause neuronal degeneration. Using site-directed mutagenesis, we have perturbed this region and will report on the significance of changes for the degeneration mechanism and the normal function of
mec-4 in the touch receptor neurons. Bob Herman has shown that
mec-4(d) is likely to act cell autonomously to kill the touch receptor neurons. We are attempting to confirm this by examining the expression of
mec-4-1acZ fusion proteins in transformed animals. -galactosidase fusions should also prove useful in clarifying details of the degeneration mechanism. For example, the activity of the
mec-6 gene is required for
mec-4-induced degenerations to occur.
mec-6 could encode a transcription factor, required for the expression of
mec-4. Alternatively,
mec-6 could form a functional molecular complex with
mec-4, such as a membrane pore. The expression of
mec4-1acZ fusion proteins in a
mec-6 mutant background may help distinguish between these possibilities.