Worm Breeder's Gazette 14(5): 65 (February 1, 1997)
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.
Department of Molecular Genetics Albert Einstein College of Medicine 1300 Morris Park Ave. Bronx, NY 10461
The mab-18 gene product is a putative transcription factor consisting of the homeodomain plus the carboxy-terminal, serine-threonine-rich transcriptional activation domain of the C. elegans Pax-6 homolog (1). mab-18 is one part of a complex locus that also includes the gene vab-3. vab-3 mutations primarily affect morphogenesis of the head (2). mab-18 is necessary for expression of the unique identity of ray 6 in the male tail. In mab-18 mutants, ray lineages are unaffected, but cells of ray 6 form a ray at the positon where ray 4 normally forms, and usually co-assemble with ray 4 cells to form a large, compound ray. This phenotype is interpreted as a ray 6 to ray 4 identity transformation. We have studied the expression pattern of MAB-18 by indirect immunofluorescence staining with affinity purified anti-MAB-18 antibodies (from a serum raised against a GST-MAB-18 fusion protein). We find that MAB-18 first appears in the ray 6 precursor cell R6. Staining is localized to the cytoplasm, and is relatively excluded from the nucleus. Cytoplasmic staining, and apparent nuclear exclusion, persist through the two rounds of cell division of the ray sublineage. After completion of the sublineage, when the ray cells begin to differentiate into ray neurons and support cell, staining disappears in the cytoplasm and appears in the nucleus, where it remains into adulthood. In our previous studies with a mab-18 reporter gene, we unexpectedly found the reporter was expressed during the ray sublineage of ray 8, a ray not known to be affected by mab-18 mutations. Expression of the reporter in ray 8 cells did not persist into the adult, but ended after the sublineages were completed. With the anti-MAB-18 antibodies, we find cytoplasmic staining in the ray 8 precursor cell, R8, and also in the ray 7 precursor cell, R7. As for ray 6, cytoplasmic staining persists through these sublineages. However, when the sublineages are completed, staining disappears altogether, and does not appear in the nucleus. In several other non-ray lineages where the antibodies detect MAB-18 or VAB-3, we observed only nuclear staining. We speculate that prior cytoplasmic accumulation of MAB-18 and sudden release into the nucleus might be important for the rapid differentiation of ray 6 cells. Alternatively, or in addition, regulation of nuclear entry might help direct action of MAB-18 to a single ray. One regulatory system might confine transcription of the mab-18 gene to the three rays 6, 7, and 8, while a second regulatory system triggers nuclear entry of the MAB-18 protein only in ray 6. Recently, Mann and Abu-Shaar (3) have found that nuclear entry of the homeodomain protein extradenticle in Drosophila embryonic midgut requires both dpp and wingless signals from visceral mesoderm. Thus regulated nuclear entry of homeodomain transcription factors may be a widespread phenomenon. 1: Zhang, Y., and Emmons, S.W. (1995) Nature 377, 55-59. 2: Chisholm, A.D., and Horvitz, H.R. (1995) Nature 377, 52-55. 3: Mann, R.S., and Abu-Shaar, M. (1996) Nature 383, 630-633.