Sma/Mab pathway signaling in C. elegans regulates proper body size formation. The components known to be involved in this TGF-beta-like signaling pathway consist of the ligand (
dbl-1), two receptor serine/threonine kinases (
sma-6 and
daf-4) and transcriptional regulators known as Smads (
sma-2, -3, and -4). Smads have previously been shown to translocate to the nucleus upon receptor stimulation, bind DNA and affect target gene transcription. Mutations in any of these components result in animals that are approximately 30% shorter in length than wild-type animals. Two genes isolated from EMS screens performed in our lab,
lon-1 and
kin-29, oppositely affect proper body size formation.
kin-29 is small, similar to the mutations seen in the known Sma/Mab pathway signaling components (also identified as
sns-8 by A. Lanjuin and P. Sengupta), while
lon-1 is approximately 25% longer than wild-type. We observe that
lon-1 is epistatic to Sma/Mab pathway signaling components and find that it encodes a novel protein with homology to the CRISP (cysteine rich secretory protein) family of proteins.
lon-1 is expressed in the hypodermis and is capable of regulating proper body size formation when expressed specifically in hypodermal tissues. We see that loss of pathway signaling results in an up-regulation of
lon-1 mRNA expression, suggesting that the Smads are unable to bind the
lon-1 promoter and repress its transcription. Recently, the Smads have been shown to bind tubulin; microtubule disruption then results in Smad nuclear accumulation and transcriptional regulation of a TGF-beta -dependent reporter gene. This suggests that the Smads are released from the microtubules, become phosphorylated by the receptor serine/threonine kinase and therefore translocate to the nucleus and affect target gene transcription. We hypothesize that
kin-29 is the component necessary for the release of the Smads from the microtubules. We have cloned
kin-29 and find that it encodes a serine/threonine kinase showing homology to members of the EMK (ELKL motif kinase) family. These proteins have been shown to phosphorylate MAPs (microtubule associated proteins), thereby affecting microtubule stability. We are currently investigating the role
kin-29 plays in microtubule stability and regulation of Smad activity.