Adaptor proteins play a vital role in the propagation of cellular signals. Although they lack endogenous catalytic activity, they contain a variety of protein binding modules, which enable them to promote specific and efficient interactions with their binding partners. They form integrative platforms for a variety of molecules (e.g. lipids, tyrosine kinases, cytoskeletal and signaling proteins), and thereby link and coordinate key functions such as cell growth, motility and shape determination. Our laboratory has recently cloned a novel, 130 kDa adaptor protein, named XB130, as a structural homolog of actin-filament-associated-protein (AFAP-110), a stress fiber-binding Src substrate. However, the molecular interactions and functions of this novel adaptor remained to be elucidated. To characterize the function of XB130 we asked two general questions: (1) Is XB130 involved in the signal transduction pathways of tyrosine kinases? And (2) Is XB130 capable of regulating the cytoskeleton and/or is it regulated by the cytoskeleton? To address these questions first we investigated the tissue distribution of XB130 and discovered that it is abundantly expressed in thyroid. Therefore we asked whether it is a target of the thyroid-specific tyrosine kinase, RET/PTC, a genetically rearranged, constitutively active enzyme that plays a pathogenic role in papillary thyroid cancer. We found that XB130 is a RET/PTC substrate that couples RET/PTC signaling to phosphatidylinositol 3-kinase (PI3K) activation through its phosphorylation dependent interaction with the regulatory subunit p85 of PI3K. XB130 plays an important role in PI3K signaling, as downregulation of XB130 in TPC1 papillary thyroid cancer cells, harboring the RET/PTC1 kinase, strongly reduced Akt activity and concomitantly inhibited cell cycle progression and survival in suspension. In the second part we demonstrate that XB130 is a novel Rac- and cytoskeleton-regulated protein that exhibits high affinity to lamellipodial (branched) F-actin and impacts motility and invasiveness of tumor cells. In conclusion, my work characterized a novel adaptor protein and assigned two well-defined pathophysiological functions to it in the context of thyroid cancer cells.

Adaptor proteins play a vital role in the propagation of cellular signals. Although they lack endogenous catalytic activity, they contain a variety of protein binding modules, which enable them to promote specific and efficient interactions with their binding partners. They form integrative platforms for a variety of molecules (e.g. lipids, tyrosine kinases, cytoskeletal and signaling proteins), and thereby link and coordinate key functions such as cell growth, motility and shape determination. Our laboratory has recently cloned a novel, 130 kDa adaptor protein, named XB130, as a structural homolog of actin-filament-associated-protein (AFAP-110), a stress fiber-binding Src substrate. However, the molecular interactions and functions of this novel adaptor remained to be elucidated. To characterize the function of XB130 we asked two general questions: (1) Is XB130 involved in the signal transduction pathways of tyrosine kinases? And (2) Is XB130 capable of regulating the cytoskeleton and/or is it regulated by the cytoskeleton? To address these questions first we investigated the tissue distribution of XB130 and discovered that it is abundantly expressed in thyroid. Therefore we asked whether it is a target of the thyroid-specific tyrosine kinase, RET/PTC, a genetically rearranged, constitutively active enzyme that plays a pathogenic role in papillary thyroid cancer. We found that XB130 is a RET/PTC substrate that couples RET/PTC signaling to phosphatidylinositol 3-kinase (PI3K) activation through its phosphorylation dependent interaction with the regulatory subunit p85 of PI3K. XB130 plays an important role in PI3K signaling, as downregulation of XB130 in TPC1 papillary thyroid cancer cells, harboring the RET/PTC1 kinase, strongly reduced Akt activity and concomitantly inhibited cell cycle progression and survival in suspension. In the second part we demonstrate that XB130 is a novel Rac- and cytoskeleton-regulated protein that exhibits high affinity to lamellipodial (branched) F-actin and impacts motility and invasiveness of tumor cells. In conclusion, my work characterized a novel adaptor protein and assigned two well-defined pathophysiological functions to it in the context of thyroid cancer cells.