PTPN21 and Hook3 relieve KIF1C autoinhibition and activate intracellular transport.

The kinesin-3 KIF1C is a fast organelle transporter implicated in the transport of dense core vesicles in neurons and the delivery of integrins to cell adhesions. Here we report the mechanisms of autoinhibition and release that control the activity of KIF1C. We show that the microtubule binding surface of KIF1C motor domain interacts with its stalk and that these autoinhibitory interactions are released upon binding of protein tyrosine phosphatase PTPN21. The FERM domain of PTPN21 stimulates dense core vesicle transport in primary hippocampal neurons and rescues integrin trafficking in KIF1C-depleted cells. In vitro, human full-length KIF1C is a processive, plus-end directed motor. Its landing rate onto microtubules increases in the presence of either PTPN21 FERM domain or the cargo adapter Hook3 that binds the same region of KIF1C tail. This autoinhibition release mechanism allows cargo-activated transport and might enable motors to participate in bidirectional cargo transport without undertaking a tug-of-war.

Crystallization and preliminary X-ray crystallographic analysis of human myotubularin-related protein 1.

Myotubularin-related protein 1 is a phosphatase that dephosphorylates phospholipids such as phosphatidylinositol 3-phosphate or phosphatidylinositol 3,5-bisphosphate. In this study, human MTMR1 was overexpressed in Escherichia coli, purified and crystallized at 277 K using polyethylene glycol 20,000 as a precipitant. Diffraction data were collected to 2.0 Šresolution using synchrotron radiation. The crystals belonged to space group P1, with unit-cell parameters a = 67.219, b = 96.587, c = 97.581 Å, α = 87.597, ß = 86.072, γ = 77.327°. Assuming the presence of four molecules in the asymmetric unit, the calculated Matthews coefficient value was 2.61 Å(3) Da(-1) and the corresponding solvent content was 52.9%.

Expression, purification and characterization of soluble red rooster laforin as a fusion protein in Escherichia coli.

BACKGROUND: The gene that encodes laforin, a dual-specificity phosphatase with a carbohydrate-binding module, is mutated in Lafora disease (LD). LD is an autosomal recessive, fatal progressive myoclonus epilepsy characterized by the intracellular buildup of insoluble, hyperphosphorylated glycogen-like particles, called Lafora bodies. Laforin dephosphorylates glycogen and other glucans in vitro, but the structural basis of its activity remains unknown. Recombinant human laforin when expressed in and purified from E. coli is largely insoluble and prone to aggregation and precipitation. Identification of a laforin ortholog that is more soluble and stable in vitro would circumvent this issue. RESULTS: In this study, we cloned multiple laforin orthologs, established a purification scheme for each, and tested their solubility and stability. Gallus gallus (Gg) laforin is more stable in vitro than human laforin, Gg-laforin is largely monomeric, and it possesses carbohydrate binding and phosphatase activity similar to human laforin. CONCLUSIONS: Gg-laforin is more soluble and stable than human laforin in vitro, and possesses similar activity as a glucan phosphatase. Therefore, it can be used to model human laforin in structure-function studies. We have established a protocol for purifying recombinant Gg-laforin in sufficient quantity for crystallographic and other biophysical analyses, in order to better understand the function of laforin and define the molecular mechanisms of Lafora disease.

Expression of the functional carbohydrate-binding module (CBM) of human laforin.

Laforin is a human protein associated with the glycogen metabolism, composed of two structurally and functionally independent domains: a phosphatase catalytic domain and a substrate-binding module with glycogen and starch affinity. The main goal of this work is the development of a methodology for the expression of the so far poorly characterized carbohydrate-binding module (CBM) of laforin, allowing its study and development of biomedical applications. The laforin's CBM sequence was originally cloned by PCR from a human muscle cDNA library. The recombinant protein, containing laforin's CBM fused to an Arg-Gly-Asp sequence (RGD), was cloned and expressed using vector pET29a and recovered as inclusion bodies (IBs). Refolding of the IBs allowed the purification of soluble, dimeric and functional protein, according to adsorption assays using starch and glycogen. Several other experimental approaches, using both bacteria and yeast, were unsuccessfully tested, pointing towards the difficulties in producing the heterologous protein. Indeed, this is the first work reporting the production of the functional CBM from human laforin.

Detection of myotubularin phosphatases activity on phosphoinositides in vitro and ex vivo.

Phosphoinositides (PPIn) are important regulators of cellular processes like intracellular protein transport, cellular proliferation, apoptosis, and cytoskeletal organization. The amount and localization of these membrane-bound second messengers are regulated through a set of specific phospholipases, lipid kinases, and phosphatases. The elucidation of PPIn-phosphatases and their cellular function has gained much attention because phosphatase dysregulation is often associated with human genetic diseases. Our laboratory has identified the 3'-PPIn-phosphatase myotubularin 1 (MTM1) mutated in X-linked myotubular myopathy (XLMTM). In addition, a whole family of myotubularin-related proteins (MTMR1-MTMR13) has been discovered. Some of them display phosphatase activity, whereas for other family members no enzymatic activity could be detected. Nevertheless, these "dead phosphatases" myotubularins are conserved throughout evolution and probably exert regulatory function by heteromeric interaction with active phosphatase members. It was shown that MTM1 and related phosphatases act on PtdIns3P and PtdIns(3,5)P2; both PPIn species are important regulators of endocytic pathways. We describe two methods to determine phosphatase activity and substrate specificity of myotubularins. One is an immunoprecipitation-phosphatase assay, testing the activity of myotubularin immunoprecipitated from overexpressing cells on artificial PPIn. The other method analyzes phosphatase activity indirectly ex vivo in transiently transfected mammalian cells. The presence and subcellular localization of the myotubularin substrate PtdIns3P were determined using a specific binding domain (2xFYVE) produced recombinantly as a biosensor.