Endofin is required for HD-PTP and ESCRT-0 interdependent endosomal sorting of ubiquitinated transmembrane cargoes

Internalized and ubiquitinated signaling receptors are silenced by their intraluminal budding into multivesicular bodies aided by the endosomal sorting complexes required for transport (ESCRT) machinery. HD-PTP, an ESCRT protein, forms complexes with ESCRT-0, -I and -III proteins, and binds to Endofin, a FYVE-domain protein confined to endosomes with poorly understood roles. Using proximity biotinylation, we showed that Endofin forms a complex with ESCRT constituents and Endofin depletion increased integrin α5-and EGF-receptor plasma membrane density and stability by hampering their lysosomal delivery. This coincided with sustained receptor signaling and increased cell migration. Complementation of Endofin- or HD-PTP-depleted cells with wild-type Endofin or HD-PTP, but not with mutants harboring impaired Endofin/HD-PTP association or cytosolic Endofin, restored EGFR lysosomal delivery. Endofin also promoted Hrs indirect interaction with HD-PTP. Jointly, our results indicate that Endofin is required for HD-PTP and ESCRT-0 interdependent sorting of ubiquitinated transmembrane cargoes to ensure efficient receptor desensitization and lysosomal delivery.

The dead phosphatases society: a review of the emerging roles of pseudophosphatases

Phosphatases are a diverse family of enzymes, comprising at least 10 distinct protein folds. Like most other enzyme families, many have sequence variations that predict an impairment or loss of catalytic activity classifying them as pseudophosphatases. Research on pseudoenzymes is an emerging area of interest, with new biological functions repurposed from catalytically active relatives. Here, we provide an overview of the pseudophosphatases identified to date in all major phosphatase families. We will highlight the degeneration of the various catalytic sequence motifs and discuss the challenges associated with the experimental determination of catalytic inactivity. We will also summarize the role of pseudophosphatases in various diseases and discuss the major challenges and future directions in this field.

Structure and functions of His domain protein tyrosine phosphatase in receptor trafficking and cancer

Cell surface receptors trigger the activation of signaling pathways to regulate key cellular processes, including cell survival and proliferation. Internalization, sorting, and trafficking of activated receptors, therefore, play a major role in the regulation and attenuation of cell signaling. Efficient sorting of endocytosed receptors is performed by the ESCRT machinery, which targets receptors for degradation by the sequential establishment of protein complexes. These events are tightly regulated and malfunction of ESCRT components can lead to abnormal trafficking and sustained signaling and promote tumor formation or progression. In this review, we analyze the modular domain organization of the alternative ESCRT protein HD-PTP and its role in receptor trafficking and tumorigenesis.

Molecular basis of interactions between SH3 domain-containing proteins and the proline-rich region of the ubiquitin ligase Itch

The ligase Itch plays major roles in signaling pathways by inducing ubiquitylation-dependent degradation of several substrates. Substrate recognition and binding are critical for the regulation of this reaction. Like closely related ligases, Itch can interact with proteins containing a PPXY motif via its WW domains. In addition to these WW domains, Itch possesses a proline-rich region (PRR) that has been shown to interact with several Src homology 3 (SH3) domain-containing proteins. We have previously established that despite the apparent surface uniformity and conserved fold of SH3 domains, they display different binding mechanisms and affinities for their interaction with the PRR of Itch. Here, we attempt to determine the molecular bases underlying the wide range of binding properties of the Itch PRR. Using pulldown assays combined with mass spectrometry analysis, we show that the Itch PRR preferentially forms complexes with endophilins, amphyphisins, and pacsins but can also target a variety of other SH3 domain-containing proteins. In addition, we map the binding sites of these proteins using a combination of PRR sub-sequences and mutants. We find that different SH3 domains target distinct proline-rich sequences overlapping significantly. We also structurally analyze these protein complexes using crystallography and molecular modeling. These structures depict the position of Itch PRR engaged in a 1:2 protein complex with β-PIX and a 1:1 complex with the other SH3 domain-containing proteins. Taken together, these results reveal the binding preferences of the Itch PRR toward its most common SH3 domain-containing partners and demonstrate that the PRR region is sufficient for binding.

Multiple Src Homology 3 Binding to the Ubiquitin Ligase Itch Conserved Proline-Rich Region

Itch is a member of the C2-WW-HECT (CWH) family of ubiquitin ligases involved in the control of inflammatory signaling pathways, several transcription factors, and sorting of surface receptors to the degradative pathway. In addition to these common domains, Itch also contains a conserved proline-rich region (PRR) allowing its interaction with Src homology 3 (SH3) domain-containing proteins. This region is composed of 20 amino acids and contains one consensus class I and three class II SH3-binding motifs. Several SH3 domain-containing partners have been shown to recognize the Itch PRR, but their binding properties have been poorly defined. Here we compare a subset of endocytic SH3 domain-containing proteins using bioluminescence resonance energy transfer, isothermal titration calorimetry, and pull-down assays. Results indicate that Endophilin is a high-affinity binding partner of Itch both in vivo and in vitro, with a calculated KD placing this complex among the highest-affinity SH3 domain-mediated interactions reported to date. All of the SH3 domains tested here bind to Itch with a 1:1 stoichiometry, except for β-PIX that binds with a 2:1 stoichiometry. Together, these results indicate that Itch PRR is a versatile binding module that can accommodate several different SH3 domain-containing proteins but has a preference for Endophilin. Interestingly, the catalytic activity of Itch toward different SH3 domain-containing proteins was similar, except for β-PIX that was not readily ubiquitylated even though it could interact with an affinity comparable to those of other substrates tested.

Cellular LITAF interacts with frog virus 3 75L protein and alters its subcellular localization

Iridoviruses are a family of large double-stranded DNA (dsDNA) viruses that are composed of 5 genera, including the Lymphocystivirus, Ranavirus, Megalocytivirus, Iridovirus, and Chloriridovirus genera. The frog virus 3 (FV3) 75L gene is a nonessential gene that is highly conserved throughout the members of the Ranavirus genus but is not found in other iridoviruses. FV3 75L shows high sequence similarity to a conserved domain found in the C terminus of LITAF, a small cellular protein with unknown function. Here we show that FV3 75L localizes to early endosomes, while LITAF localizes to late endosomes/lysosomes. Interestingly, when FV3 75L and LITAF are cotransfected into cells, LITAF can alter the subcellular localization of FV3 75L to late endosomes/lysosomes, where FV3 75L then colocalizes with LITAF. In addition, we demonstrated that virally produced 75L colocalizes with LITAF. We confirmed a physical interaction between LITAF and FV3 75L but found that this interaction was not mediated by two PPXY motifs in the N terminus of LITAF. Mutation of two PPXY motifs in LITAF did not affect the colocalization of LITAF and FV3 75L but did change the location of the two proteins from late endosomes/lysosomes to early endosomes.

SIMPLE/LITAF expression induces the translocation of the ubiquitin ligase itch towards the lysosomal compartments

LITAF is a small cellular protein with an unknown function. The C-terminus of LITAF contains a highly conserved domain termed the SIMPLE-like domain (SLD), while the N-terminus contains two PPXY motifs that mediate protein-protein interactions with WW-domain containing proteins. LITAF also harbors two endosome/lysosome targeting sequences at its C-terminus, but there has been conflicting reports regarding its intracellular localization. Here, we demonstrate that LITAF is localized to the late endosome/lysosomal compartment in a variety of cell lines. We also show that Itch, a WW-domain containing protein, and LITAF strongly interact and that this interaction depends on the two PPXY motifs in the N-terminus of LITAF. Interestingly, co-expression of LITAF with Itch induces major changes in Itch intracellular localization, bringing Itch from the trans-Golgi network to lysosomes. We show that this re-localization is dependent upon the interaction with the PPXY sequences of LITAF, since disruption of these binding motifs completely abrogates Itch re-localization.

A simple isokinetic cycle for measurement of leg muscle function

The measurement of net pedaling torque during isokinetic cycling allows for the evaluation of leg muscle strength and work capacity over fixed time intervals. However, the expense and difficulty of constructing an isokinetic cycle have limited the widespread application of this useful research tool. We have modified a simple commercially available isokinetic cycle that uses hydraulics to maintain pedaling velocity. A strain gauge on the flywheel axle strut measures the torsion on the strut caused by pedaling. To evaluate this device, seven healthy subjects (3 males and 4 females) were each tested twice at 60, 90, and 120 rpm for peak power during a 10-s sprint and at 100 rpm for total work performed during a 30-s sprint. These results were compared with predicted values for age, height, and sex developed on a more complicated isokinetic cycle. Subjects also performed a progressive cycle ergometry test. For the group, peak power was 97.30 +/- 12.64% of predicted (males 883.70 +/- 202.76 W; females 657.00 +/- 66.42 W) and work output was 107.70 +/- 15.75% of predicted (males 15.50 +/- 2.85 kJ; females 11.70 +/- 2.17 kJ), whereas maximal progressive exercise capacity was 126.40 +/- 25.84% (males 245.30 +/- 56.58 W; females 212.30 +/- 35.49 W). The relatively lower work values generated on this cycle (compared with the maximal progressive exercise capacity) can be attributed to the location of the strain gauge, resulting in measurement of effective work output on the flywheel.(ABSTRACT TRUNCATED AT 250 WORDS)