Worm Breeder's Gazette 14(3): 56 (June 1, 1996)
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.
Dept. of Biological Sciences University of Alberta Edmonton, Alberta CANADA
Shortly after we reported the cloning of unc-119 (Genetics
141:977-988, 1995), a predicted protein of very similar sequence
appeared in the databases. Higashide et al. (J. Biol. Chem.
271:1797-1804, 1996) identified an apparently novel protein expressed in
the human retina. The cDNA for HRG4 (human retinal gene) was recovered
in a search for retina-specific cDNAs using a subtractive hybridization
approach. They demonstrated that the mRNA for the rat homolog RRG4
accumulates specifically in the retina around postnatal day 5 (the time
of photoreceptor cell differentiation) and remains present through
adulthood. Furthermore, in-situ hybridization showed that the message
is specific for the rod and cone cells in the retina.
The similarities between UNC-119 and HRG4 are striking (see
Figure 1). The sizes are very similar (219 a.a. for UNC-119 vs. 240
a.a. for HRG4) and there is little homology at the very amino termini
where the !extra! amino acids appear in an ideal alignment. The
similarities reside in two extended domains that split the ORFs in two.
The first region A shows 56% identity, and the B region shows 67%
identity. The lack of conservation between the two regions coincides
with the portion of the C. briggsae UNC-119 homolog that shows weakest
similarity to C. elegans. The conserved regions extend over many groups
of contiguous residues, the longest being in the carboxy terminus, which
contains the conserved sequence -LVMHNKADYSY-. Both ORFs lack regions
with similarity to other published sequences.
In an effort to determine the degree of functional
interchangeability with HRG4, we used RT-PCR on total RNA from human
retina (a generous gift from the lab of Ian MacDonald, U of A) to obtain
the majority of the coding region. We cloned this fragment into a
vector that lacks the C. elegans ORF, but which retains the promoter and
3! flanking sequences (pDP#MM103; see Figure 2). The transgene appears
to completely rescue the movement defect of unc-119(ed3op) mutants,
consistent with the conservation of the function of this gene through
evolution.
If HRG4 can substitute for UNC-119, what do these proteins do?
And of all the neural tissues in a human being, why is it only found in
the retina? A clue may come from preliminary work using unc-119::GFP
fusions to mark the nervous system. Animals mutant for unc-119 show an
apparent neuron outgrowth defect, which is most noticeable in
circumferential processes: These seem to have multiple branches and
morphologies consistent with a disruption in outgrowth. Perhaps UNC-119
has a role in the correct assembly of the cytoskeleton, such that
mutants manifest a phenotype resulting from an inability to favor proper
growth cone projections.
From the nematode findings, we can predict that a mouse or human
disrupted for this gene would have disorganized photoreceptors, and
hence be blind. Thus far, however, HRG4 has not been mapped or
implicated in a human disease.
[two small figures in original]