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In the nematode Caenorhabditis elegans, microtubules with 15 protofilaments are a specialized feature of six touch-receptor neurons; microtubules found in other C. elegans neurons have 11 protofilaments. Mutations in the gene mec-7 result in touch-insensitive animals whose touch cells lack the 15-protofilament microtubules. We have characterized 54 mutations in the mec-7 gene. The absence of mec-7 activity results selectively in the recessive loss of touch sensitivity. Partial loss-of-function alleles result in a partial loss of touch sensitivity. Dominant mutations, which are isolated at an unusually high proportion, may encode abnormal products. We have cloned the mec-7 gene; it encodes a 13-tubulin which is 90-93% identical to vertebrate 13-tubulin. Our results are consistent with the hypothesis that tubulin heterogeneity contributes to the formation of structurally and functionally distinct sets of microtubules.

The dwarfin protein family has been genet- Transforming growth factor-f3 (TGF-P3) is a multifunctional polypeptide hormone that elicits a wide range of cellular effects, including inhibition of cellular proliferation and transcriptional activation of specific target genes (1, 2). The TGF-,3 signal is initiated through a heteromeric transmembrane kinase complex of type I and type II receptors (3-6). A potential mechanism of activation for the heteromeric TGF-,B receptor complex has been proposed (7). Within the heteromeric complex, the type II receptor phosphorylates the type I receptor, and activation of the type

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