Lfc is a guanine nucleotide exchange factor (GEF) that activates the small GTPase RhoA, and its GEF activity is tightly regulated through protein-protein interactions, phosphorylation, and cellular localization. Lfc is anchored to microtubules through its interaction with the dynein light chain Tctex-1, which results in inhibition of Lfc's GEF activity. Here we present a crystallographic structure of Tctex-1 in complex with Lfc with residues 143-155 of Lfc bound at the Tctex-1 dimer interface. Structural alignment of our structure with Tctex-1 in complex with the dynein intermediate chain (DIC) shows the binding site of the DIC peptide and Lfc substantially overlap. Biochemical evidence, NMR perturbations assays and intrinsic fluorescence provide structural validation and support an extension of the Lfc binding site to the α-helices that may accommodate additional contact points with Tctex-1. We postulate a potential mechanism for Lfc’s recruitment to the microtubules through a tripartite complex with Tctex-1 and DIC.

Lfc is a guanine nucleotide exchange factor (GEF) that activates the small GTPase RhoA, and its GEF activity is tightly regulated through protein-protein interactions, phosphorylation, and cellular localization. Lfc is anchored to microtubules through its interaction with the dynein light chain Tctex-1, which results in inhibition of Lfc's GEF activity. Here we present a crystallographic structure of Tctex-1 in complex with Lfc with residues 143-155 of Lfc bound at the Tctex-1 dimer interface. Structural alignment of our structure with Tctex-1 in complex with the dynein intermediate chain (DIC) shows the binding site of the DIC peptide and Lfc substantially overlap. Biochemical evidence, NMR perturbations assays and intrinsic fluorescence provide structural validation and support an extension of the Lfc binding site to the α-helices that may accommodate additional contact points with Tctex-1. We postulate a potential mechanism for Lfc’s recruitment to the microtubules through a tripartite complex with Tctex-1 and DIC.

During development of the mammalian nervous system, neural stem cells generate neurons first and glia second, thereby allowing the initial establishment of neuronal circuitry, and subsequent matching of glial numbers and position to that circuitry. Multiple molecular mechanisms act in concert to control neural precursor expansion prior to neurogenesis, and to allow for an exponential generation of neurons while ensuring the maintenance of sufficient precursors to produce later-born neurons, glial cells and adult neural stem cells. Throughout cortical development, these processes are regulated in part by the precursors’ environment as well as intrinsic changes in precursors and their modes of division, which regulate the fate of daughter cells and the balance between self-renewal and differentiation. In the first part of this thesis, the protein tyrosine phosphatase SHP-2 was identified as a novel signaling protein that regulates the neurogenic to gliogenic switch by potentiating neurogenic signals and suppressing gliogenic signals until the appropriate developmental time point for astrogenesis, providing one mechanism whereby precursors integrate conflicting environmental cues. A Noonan Syndrome (NS)-associated activated SHP-2 mutation causes perturbations in neural cell genesis, which may contribute to the mild mental retardation and learning disabilities observed in NS patients. In the second part of this thesis, a novel Rho-regulatory pathway which includes the Rho-GEF Lfc and its negative regulator Tctex-1 were also found to regulate neurogenesis, potentially by directing mitotic spindle orientation during precursor divisions, thereby regulating the symmetric and asymmetric nature of radial precursor divisions.

During development of the mammalian nervous system, neural stem cells generate neurons first and glia second, thereby allowing the initial establishment of neuronal circuitry, and subsequent matching of glial numbers and position to that circuitry. Multiple molecular mechanisms act in concert to control neural precursor expansion prior to neurogenesis, and to allow for an exponential generation of neurons while ensuring the maintenance of sufficient precursors to produce later-born neurons, glial cells and adult neural stem cells. Throughout cortical development, these processes are regulated in part by the precursors’ environment as well as intrinsic changes in precursors and their modes of division, which regulate the fate of daughter cells and the balance between self-renewal and differentiation. In the first part of this thesis, the protein tyrosine phosphatase SHP-2 was identified as a novel signaling protein that regulates the neurogenic to gliogenic switch by potentiating neurogenic signals and suppressing gliogenic signals until the appropriate developmental time point for astrogenesis, providing one mechanism whereby precursors integrate conflicting environmental cues. A Noonan Syndrome (NS)-associated activated SHP-2 mutation causes perturbations in neural cell genesis, which may contribute to the mild mental retardation and learning disabilities observed in NS patients. In the second part of this thesis, a novel Rho-regulatory pathway which includes the Rho-GEF Lfc and its negative regulator Tctex-1 were also found to regulate neurogenesis, potentially by directing mitotic spindle orientation during precursor divisions, thereby regulating the symmetric and asymmetric nature of radial precursor divisions.