Worm Breeder's Gazette 14(1): 45 (October 1, 1995)

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.

Biochemical Analysis of Transmembrane Topology of MEC-4--A Channel Subunit Involved in Mechanosensory Neuron Function and Neurodegeneration

C-C. Lai,, J. Xue,, and M. Driscoll.

Department of Molecular Biology and Biochemistry, Rutgers University, New Brunswick, NJ 08855

mec-4 encodes a protein related to mec-10, DEG-1 and subunits of the
mammalian amiloride-sensitive epithelial Na+ channel.  mec-4 and mec-10
are co-expressed in the six touch receptor neurons and are postulated to
encode subunits of a mechanosensory ion channel.  This working
hypothesis is exciting because, unlike voltage-gated and ligand-gated
channels, mechanically-gated channels (those opened by membrane stretch
or physical deflection) have not yet been characterized at the molecular
level.  mec-4 and related family members have a second interesting
feature--specific dominant gain-of-function alleles induce swelling and
degeneration of the neurons in which they are expressed.  We are
interested in understanding the normal and inappropriate activity of this
newly identified channel class.

Biophysical studies support that tension applied to mechanosensitive ion
channels is required for their regulated opening and Chalfie and
colleagues have identified extracellular matrix and intracellular proteins
that may interact with the touch receptor channel (see wm95 p73).  Thus,
the determination of channel subunit topology is important for defining
target regions for interaction with the cytoskeleton or extracellular
matrix, contacts likely to be essential for mechanotransduction.  MEC-4
has two hydrophobic domains that are theoretically capable of spanning the
cell membrane.  The predicted coding sequence does not include a
hydrophobic N-terminal signal sequence.  As a consequence, MEC-4 is
expected to have an intracellular N-terminal domain, a large extracellular
domain that includes two Cys-rich regions and a short intracellular C-
terminal domain.  We have performed biochemical experiments that
support this overall transmembrane topology.  We raised antibodies
against 4 MEC-4 regions:  the N-terminal domain, two stretches in the
Cys-rich region, and the C-terminal domain.  When we translate mec-4 in
vitro in the presence of microsomal membranes we detect a glycoslyated
product, which when treated with endoglycosidase H, yields a protein of
the size predicted for MEC-4.  In microsomal ER membranes, domains
that would normally project into the extracellular space project into the
ER lumen, where they are accessible to modifying enzymes such as those
that add sugar moieties and domains that normally project into the
cytoplasm project outside the lumen and are accessible to enzymes added to
the reaction, such as proteases.  When we translate mec-4 in vitro in the
presence of microsomes and then add proteinase K to the reaction mix, the
size of the reaction product is decreased by approximately 20 kD, a value
consistent with proteolytic removal of the predicted 15 kD N-terminal and
3.4 kD C-terminal domains.  The protease-resistant fragment is
glycosylated and can be recognized by antibodies directed against the Cys-
rich region but is not recognized by anti-N-and C-terminal antibodies.
These results support that the N- and C-termini are normally intracellular
and that no other domain of MEC-4 projects into the intracellular space.
Results of gene fusion experiments support this topology--fusion of lacZ
to the N-and C-termini of MEC-4 are active for b-galactosidase activity
wereas fusions to the extracellular domain are not.

The facts that mec-4 and mec-10 are co-expressed in the touch receptor
neurons, that mec-10(d)-induced degeneration depends on mec-4, and that
the mammalian channel includes multiple homologous subunits support
that mec-10 and mec-4 are part of the same channel complex.  In
preliminary experiments on channel assembly, we have demonstrated that
MEC-4 and MEC-10 can be co-immunoprecipitated when expressed in the
presence of microsomes, providing the first physical evidence that these
proteins associate in the endoplasmic reticulum.