Activin, a member of the TGF-b superfamily, controls a variety of physiological events from the cell cycle to reproductive health. Activin activates the Smad signaling pathway through ActRII/B and ALK4 receptors. The regulation of activin signaling is carried out in part by the inhibitor protein follistatin. The mechanistic details of follistatin function remain elusive due to the presence of multiple follistatin isoforms with varying affinities for activin and heparin. Moreover, the co-receptor cripto is the sole factor that differentiates the use of activin receptors in this pathway from another TGF-b member, nodal. It is intriguing that nodal, but not activin, requires cripto as an accessory protein in signaling. Cripto plays multiple roles in development and cancer by functioning at the crossroads of multiple signaling pathways. Cripto functionally interacts with several ligands and receptors, sometimes with quite different cellular outcomes. Signaling by activin and nodal can therefore be regulated by both negative (follistatin:activin) and positive (cripto:nodal) factors. In order to characterize the molecular details of activin regulation by follistatin and cripto, structural and biophysical characterizations of a number of proteins and protein complexes have been performed. Structural evaluation of the activin:follistatin 315 complex reveals a potential linkage between activin and heparin binding to follistatin 315. Subsequent biophysical assessments provide support for this linkage and suggest a novel mechanism of turnover for bioneutralized follistatin 315 complexes utilizing enhanced heparin affinity. Expression and characterization of ALK4 and cripto proteins demonstrates a pH dependent dissociation of complexes between ALK4 and the CFC domain of cripto. Furthermore, quantitative binding analyses of these interactions suggest that the EGF-like domain of cripto inhibits ALK4 binding to the CFC domain. These results suggest a potential cooperative assembly of nodal signaling components and that the selective use of cripto helps control the sharing of ALK4 between activin and nodal. From insights into heparin binding by various follistatin species to the effects of pH on cripto complexes, these studies highlight the powerful effects that the extracellular environment has on activin regulatory proteins.

Last modified

• 08/07/2018

Creator

• Thomas F Lerch

DOI

• https://doi.org/10.21985/N28X3F

Subject

• Interdepartmental Biological Sciences Program

Keyword

• Biology
• Molecular

Date created

• 2007-08-22

Resource type

• Dissertation

Rights statement

• In Copyright