Worm Breeder's Gazette 13(1): 38 (October 1, 1993)
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.
A characteristic of the touch receptor neurons is the presence of large diameter microtubules (15 protofilaments, instead of 11) in their axons. Mutants of mec-7 and mec-12 specifically lack} these microtubules and as a result, are touch insensitive (Dev. Biol. 82:358; Science 243:1027). We now know that mec- 7 encodes for a novel beta-tubulin whose expression is confined to the touch neurons (Genes and Dev. 3:870; EMBO J. 1 1:2885).
The phenotypic similarity between mec-12 and mec-7 suggests that mec-12 encodes the alpha-tubulin counterpart of the touch neuron microtubules. Indeed, hybridization of the MRC YAC grid with a probe made from the NWA3 alpha-tubulin gene, reveals the presence of another member of the family on chromosome III, in the mec-12 region. Immunostaining with 6-11B-1 also supports the alpha-tubulin hypothesis. This monoclonal antibody, which specifically recognizes [ lys-40 ]- acetylated alpha-tubulin in various organisms, brightly stains C. elegans touch neurons and PVR (J. Neurosc. 9:2963); it also stains (less intensely) the axons of most other neurons. We have now found that mec-12 ( u76 )animals are defective in 6-11B-1 staining.
The chromosome III alpha-tubulin is present on the overlapping YACs Y6H4 , Y6G9 ,and Y53G8 from contig 377. Of all the cosmids in that region, only C56F12 gave a positive hybridization signal. By germline transformation using C56F12 DNA, we have rescued touch sensitivity in mec-12 ( u76 )and in mec-12 ( e1605 ).Moreover, rescued mec-12 ( u76 )now stain with 6-11B-1. We are presently trying to confirm that these rescues are caused by the alpha-tubulin gene present on C56F12 .Sequencing is under way.
These results suggest that the putative mec-12 alpha-tubulin is the only [ lys-40 ]- acetylated isotype of the family (there are probably 5 alpha-tubulin genes in C. elegans; 2 of them have been characterized and do not encode for a lysine at position 40 [S. Siddiqui, pers. comm.]). If true, the 611B-1 staining pattern then predicts that mec-12 is expressed in the entire nervous system, as opposed to mec-7 .In this scheme, mec-12 could possibly assemble into both normal (as in most neurons) and large diameter microtubules (as in the touch neurons). The mec-7 beta-tubulin, on the other hand, is believed to be solely a large diameter microtubule component.
No exact role is known for microtubule acetylation, but it is often correlated with an increased stability of the polymer. In C. elegans, this postranslational modification does not appear to be essential, as the only obvious phenotype of mec-12 is touch insensitivity.