Worm Breeder's Gazette 13(4): 86 (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.

Different mec-9 Transcripts Are Expressed in Touch Cells and Ventral Cord Motor Neurons.

Hongping Du, Marty Chalfie

Dept. of Biol. Sci., Columbia Univ, New York

  mec-9 mutants are touch insensitive but their touch cells have no morphological
defects. A 10 kb genomic fragment rescues the mec-9 phenotype and identifies two
overlapping transcripts of 2 kb and 3 kb. The 3 kb transcript differs from the 2 kb
transcript mainly in the 1 kb region at the 5' end. The 3 kb transcript level is more
than 15 fold reduced in mec-3 mutants ( mec-3 is needed for touch receptor
differentiation), while the 2 kb transcript level was unchanged, indicating that the long
transcript is specific to the touch receptors. The two transcripts are differentially
expressed. A MEC-9 -GFPfusion protein for the long transcript, produced by
introducing gfp into the region encoding the 5' 1 kb portion of the 3 kb transcript, is only
observed in the six touch receptor neurons and the PVD cells. Both lacZ and GFP
fusions made from the short transcript showed it to be expressed in over 30 ventral cord
motor neurons and 14 cells in the head. A lacZ fusion made after a common region in
the two transcripts gave lacZ expression in both groups of cells. We believe that
independent promoters control the production of the two mec-9 transcripts.
  We obtained an almost full-length cDNA from the Chris Martin library and used
RACE with anchored primers to detect SL1 and SL2 splice leaders (in equal
proportions). Primers from the genomic DNA and an SL1 primer also allowed us to
isolate the 1 kb fragment at the 5' end of the larger transcript. Starting from the 5' end,
the large protein has two Kunitz-type serine protease inhibitor domains, three
EGF-like repeats (two of which are of the calcium-binding type), three additional
Kunitz-type domains, three EGF-like repeats of the non-calcium-binding type, and a
glutamic acid-rich domain. The short protein has the last two Kunitz domains, the
downstream EGF-like repeats, and the glutamic-acid rich domain. All these
domains have only been found in proteins in the extracellular space, so we think it is
likely that the mec-9 proteins are secreted. We have not, however, found a signal
sequence in either coding region.
  Agrin, a protein needed in vertebrates for the aggregation of acetylcholine
receptors, has similar EGF-like repeats to those in both the long and short transcripts
and also the serine protease inhibitor domains (albeit of the Kazal rather than the
Kunitz type). We do not know if either of the mec-9 transcripts encode proteins with a
similar function.
  The expression of the short transcript in the ventral cord neurons was
surprising, since none of the mec-9 mutants are Unc. One possibility we are testing
now is that all of our existing mec-9 mutations only affect the 3 kb transcript. We have
characterized 11 mec-9 alleles with mutations in the region specific to the larger
protein. In addition to two Tc1 insertions, an EMS-derived deletion, and three
nonsense mutations, there are five missense mutations in the calcium-binding type of
EGF-like repeats. These latter mutations suggest that the EGF-like domains are
important for mec-9 function. We are sequencing five additional alleles and have yet
to find mutations in the large transcript-specific region (only the most 5' and 3' parts of
the region have not been sequenced). It is possible that these mutations are in common
exons for the two transcripts. In addition we have looked for additional mec-9
loss-of-function alleles by screening for noncomplementing mutations. We obtained
five new mec-9 alleles (26,000 haploid genomes screened). All five mutations produced
only the Mec phenotype, so it is possible that the short transcript is not essential.