MEC-12, an α-tubulin required for touch sensitivity in C. elegans

Abstract

mec-12 is one of a dozen genes required for touch receptor neuron function in Caenorhabditis elegans. Some mec-12 mutants (mechanosensory-defective) lack the large-diameter microtubules that are characteristic of these neurons (15 protofilaments, as opposed to 11). Mutants of mec-7, a alpha-tubulin encoding gene, have a similar phenotype. We have identified the nature of mec-12 by germline transformation rescue and characterization of a point mutation. Sequence analysis of the mec-12 encoded product (MEC-12) indicates that it corresponds to a novel C. elegans alpha-tubulin. MEC-12 is the only identified C. elegans alpha-tubulin that contains a lysine at position 40, a known site of post-translational acetylation. Some mec-12 mutations eliminate microtubule acetylation as assayed immunocyto-chemically; phenotypic rescue using a MEC-12 variant lacking the lysine-40 showed that acetylation is not required for MEC-12 activity. Although functionally needed only in the touch neurons, mec-12 is expressed in several other neuron types. These results support the notion that tubulin isotype diversity contributes to the formation of distinct classes of microtubules; 15-protofilament microtubule assembly requires MEC-12 alpha-tubulin and MEC-7 beta-tubulin, which are both highly expressed in the touch receptor neurons. MEC-12 is the first reported alpha-tubulin isotype that appears to be required in a single class of neuronal microtubules.

Mechanosensation

“…Their processes are filled with unusual, 15-protofilament (15-p) microtubules that are cross-linked to one another and arrayed such that their distal ends are in close apposition to the cell membrane (Chalfie and Thomson, 1979). The 15-p microtubules are unique to the touch receptor neurons (Chalfie and Sulston, 1981) and arise from tubulins (MEC-7 and MEC-12) abundant in touch receptor neurons (Fukushige et al, 1999;Savage et al, 1989). Ultrastructural specializations are not restricted to the cytoskeleton, however.…”

Sexually dimorphic architecture and function of a mechanosensory circuit in C. elegans

“…First, targeting mec-12 by using both RNAi or a mec-12 mutant, reduced tail mechanosensation only in males (Figure 1D-E). mec-12 (expressed in PHA, PHB and PHC, Figure 1C) encodes an alpha-tubulin protein, is one of several genes required for touch receptor neuron (TRN) function in C. elegans and is specifically responsible for generating 15-protofilament microtubules in TRNs 37 .…”

“…We found four genes that function sex-specifically in the circuit: mec-12 and tmc-1 in sensory cells, and glr-1 and nmr-1 in interneurons. Previous reports indicate that the alpha-tubulin MEC-12 is required with the beta-tubulin MEC-7 for 15-protofilament microtubule assembly in touch receptor neurons (TRNs), responsible for the transduction of gentle touch 37,48 . Our data suggests that MEC-12 functions through PHA/PHB to mediate tail mechanosensation in males, but whether this function requires MEC-7 remains to be determined.…”

The Expression and Function of Tubulin Isotypes in Caenorhabditis elegans

“…Both mec-12 and mec-7 are required for the mechanosensory functions of TRNs ( Chalfie and Au 1989 ). It is worth noting that mec-7 is also expressed in a few other neurons outside of the TRNs, and mec-12 expression is found in many neurons, including motor neurons and interneurons, although their expression in the six TRNs are much higher compared to other expression ( Fukushige et al, 1999 ; Nishida et al, 2021 ). This expression pattern is supported by scRNA-seq data ( Figure 2 ).…”

“…One α-tubulin ( mec-12 ) and one β-tubulin ( mec-7 ) showed highly enriched expression in the mechanosensory neurons, also known as the touch receptor neurons (TRNs) including ALM, AVM, PLM, and PVM ( Hamelin et al, 1992 ; Fukushige et al, 1999 ). These neurons contain the special 15-protofilament microtubules, while all other neurons in C. elegans have 11-protofilament microtubules ( Chalfie and Thomson 1982 ).…”