EXTRACELLULAR PROTEINS NEEDED FOR C-ELEGANS MECHANOSENSATION

The mec-5 and mec-9 genes encode putative extracellular proteins that allow a set of six touch receptor neurons in C. elegans to respond to gentle touch. MEC-5 is a collagen made by the epidermal cells that sur round the touch cells. Mutations causing touch insensitivity affect th e Gly-X-Y repeats of this collagen. mec-9 produces two transcripts, th e larger of which is expressed in the touch cells and two PVD neurons. This transcript encodes a protein with 5 Kunitz-type protease inhibit or domains, 6 EGF-like repeats (2 of the Ca2+-binding type), and a glu tamic acid-rich region. Missense mutations causing touch insensitivity affect both the EGF-like and Kunitz domains. Since mec-9 loss of func tion mutations dominantly enhance the touch insensitive phenotype of s everal mec-5 mutations, MEC-5 and MEC-9 may interact. We propose that these proteins provide an extracellular attachment point for the mecha nosensory channels of the touch cells.